Go 1.26.0-1 doesn't include revision in microsoft_toolset_version

[dagood]

In mcr.microsoft.com/oss/go/microsoft/golang:1.26-azurelinux3.0, I'm not seeing the revision:

# GODEBUG=ms_version=1 go version
go version go1.26.0-microsoft linux/amd64

# go version -m main
main: go1.26.0
        path    command-line-arguments
        build   microsoft_systemcrypto=1
        build   microsoft_toolset_version=go1.26.0-microsoft
[...]
# go version -m $(go env GOROOT)/bin/go
/usr/local/go/bin/go: go1.26.0
        path    cmd/go
        build   microsoft_toolset_version=go1.26.0-microsoft
[...]

[dagood]

The MICROSOFT_REVISION file is packed up, but it isn't present when the build is taking place (so, not present when src/internal/buildcfg/zbootstrap.go is generated). I tested the release branch flow with a revision, but it's possible I only tested it with one of the earlier approaches to implement the feature.

go/eng/_util/cmd/build/build.go

Lines 284 to 285 in e5b716e

	microsoftRevisionDst := filepath.Join(goRootDir, "MICROSOFT_REVISION")
	if err := copyFile(microsoftRevisionDst, filepath.Join(rootDir, "MICROSOFT_REVISION")); err != nil {

I think this just needs to be moved earlier in the process so MICROSOFT_REVISION is present during the build.