Go: Add diagnostic for 1.21 toolchain error

[mbg]

Summary

We keep seeing customers using advanced workflows who run into problems with their Go 1.21 projects and CodeQL in GHA environments:

• The current default version of Go on GHA images is still 1.20.
• We use the system Go toolchain under the hood to perform the build.
• Consequently, Go 1.21 projects fail to build if Go 1.21 is not explicitly installed.

This PR introduces a new diagnostic which is emitted if the unknown directive: toolchain error is detected to advise users that a new system version of Go is required along with a link to instructions on how to do that in a GHA workflow.

[mbg]

It's tricky to build an integration test for this, since it depends on having a particular Go version in the environment, but I have tested this locally with PATH="/opt/homebrew/opt/go@1.20/bin:$PATH" codeql database create ./codeql-db --language=go --search-path=[path to]/semmle-code/target/intree/codeql-go/go in a folder with a go.mod that contains a toolchain directive and the diagnostic is emitted as expected:

{
  "timestamp": "2023-09-07T11:08:57.439Z",
  "source": {
    "id": "go/autobuilder/newer-system-go-version-required",
    "name": "The Go version installed on the system is too old to support this project",
    "extractorName": "go"
  },
  "markdownMessage": "At least Go version `1.21.0` is required to build this project, but the version installed on the system is older. [Install a newer version](https://github.com/actions/setup-go#basic).",
  "severity": "error",
  "visibility": {
    "statusPage": true,
    "cliSummaryTable": true,
    "telemetry": true
  }
}

As with other diagnostics, this will only currently show up for CodeQL runs on the default repository branch. I think it would probably be useful to emit something to the log as well.

Furthermore, there are multiple places where this error occurs and we could catch it when parsing go.mod files too. We don't currently scan the output of the go command invocations, but we could search the go.mod file for a toolchain directive manually and check that the Go version is at least 1.21.

Full build log

Initializing database at .../go-test/codeql-db.
Running build command: [.../codeql/semmle-code/target/intree/codeql-go/go/tools/autobuild.sh]
[2023-09-07 12:08:57] [build-stderr] 2023/09/07 12:08:57 Autobuilder was built with go1.21.0, environment has go1.20.8
[2023-09-07 12:08:57] [build-stderr] 2023/09/07 12:08:57 LGTM_SRC is .../go-test
[2023-09-07 12:08:57] [build-stderr] 2023/09/07 12:08:57 Found go.mod, enabling go modules
[2023-09-07 12:08:57] [build-stderr] go: errors parsing go.mod:
[2023-09-07 12:08:57] [build-stderr] .../go-test/go.mod:5: unknown directive: toolchain
[2023-09-07 12:08:57] [build-stderr] 2023/09/07 12:08:57 Running /opt/homebrew/opt/go@1.20/bin/go failed, continuing anyway: exit status 1
[2023-09-07 12:08:57] [build-stderr] 2023/09/07 12:08:57 Failed to run `go mod tidy -e`
[2023-09-07 12:08:57] [build-stderr] 2023/09/07 12:08:57 Unable to determine import path, as neither LGTM_INDEX_IMPORT_PATH nor GITHUB_REPOSITORY is set
[2023-09-07 12:08:57] [build-stderr] 2023/09/07 12:08:57 Build failed, continuing to install dependencies.
[2023-09-07 12:08:57] [build-stderr] 2023/09/07 12:08:57 Installing dependencies using `go get -v ./...` in `.`.
[2023-09-07 12:08:57] [build-stderr] go: errors parsing go.mod:
[2023-09-07 12:08:57] [build-stderr] .../go-test/go.mod:5: unknown directive: toolchain
[2023-09-07 12:08:57] [build-stderr] 2023/09/07 12:08:57 Running /opt/homebrew/opt/go@1.20/bin/go failed, continuing anyway: exit status 1
[2023-09-07 12:08:57] [build-stderr] 2023/09/07 12:08:57 Running extractor command '.../semmle-code/target/intree/codeql-go/go/tools/osx64/go-extractor [./...]' from directory '.'.
[2023-09-07 12:08:57] [build-stderr] 2023/09/07 12:08:57 Build flags: ''; patterns: './...'
[2023-09-07 12:08:57] [build-stderr] 2023/09/07 12:08:57 Running packages.Load.
[2023-09-07 12:08:57] [build-stderr] 2023/09/07 12:08:57 Error running go tooling: err: exit status 1: stderr: go: errors parsing go.mod:
[2023-09-07 12:08:57] [build-stderr] .../go-test/go.mod:5: unknown directive: toolchain
[2023-09-07 12:08:57] [build-stderr] 2023/09/07 12:08:57 Extraction failed: exit status 1
[2023-09-07 12:08:57] [ERROR] Spawned process exited abnormally (code 1; tried to run: [.../.local/share/gh/extensions/gh-codeql/dist/nightly/codeql-bundle-20230728/tools/osx64/preload_tracer, .../semmle-code/target/intree/codeql-go/go/tools/autobuild.sh])
A fatal error occurred: Exit status 1 from command: [.../semmle-code/target/intree/codeql-go/go/tools/autobuild.sh]

[smowton]

Looks good to me. Local testing is sufficient IMO.