build asset JSON: handle go.*.src.tar.gz files, improve errors

[dagood]

• Fix issue found in testing build for Create source archive during CI go#408
• For Create source archive build artifacts go#398

Check for .src.tar.gz and avoid trying to parse it as a binary build, because it will fail. Also improve the error message in case of failure, and store the source tarball URL in the build asset JSON to make the file more complete.

[dagood]

I'm running a test Go internal rolling build with this change vendored in here: https://dev.azure.com/dnceng/internal/_build/results?buildId=1598321&view=results.

[dagood]

@microsoft/golang-compiler PTAL
The test Go internal build succeeded, so this should be good to go.

[dagood]

I noticed this leaves a blank-ish entry in the build asset JSON because the checksum file of the source tarball isn't handled correctly:

    {
      "env": {
        "GOARCH": "",
        "GOOS": ""
      },
      "sha256": "4f1f09428fcb78786f75eff612e09f9e8271caff03e834f806aa9d967d17544d",
      "url": ""
    },

Working on it. [Done.]

https://dev.azure.com/dnceng/internal/_build/results?buildId=1598617&view=results