version: show tailscale/go toolchain git hash in version output

When built with the Tailscale Go toolchain, include the toolchain's
git revision in the version output. The non-JSON output shows the
first 10 hex digits:

  go version: go1.26.2 (tailscale/go dfe2a5fd8e)

The JSON output includes the full hash as "tailscaleGoGitHash", or
omits the field when not using tsgo.

The toolchain rev is read via a separate sync.OnceValue rather than
piggybacking on getEmbeddedInfo, because that function discards all
data when VCS fields are absent (e.g. in test binaries), while the
tailscale.toolchain.rev setting is still present.

Also add a CI-only test verifying tailscaleToolchainRev is non-empty
when built with the tailscale_go build tag.

Fixes #19374

Change-Id: Ied0b16d7aead5471d8c614c30cba8b0dcf80c691
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

version/version.go

@@ -146,6 +146,23 @@ var getEmbeddedInfo = sync.OnceValue(func() embeddedInfo {
 	return ret
 })
 
+// tailscaleToolchainRev returns the git hash of the Tailscale Go toolchain
+// used to build this binary, if any. It is read separately from getEmbeddedInfo
+// because that function discards build info when VCS fields are missing (e.g.
+// in test binaries), but the toolchain rev is still present.
+var tailscaleToolchainRev = sync.OnceValue(func() string {
+	bi, ok := debug.ReadBuildInfo()
+	if !ok {
+		return ""
+	}
+	for _, s := range bi.Settings {
+		if s.Key == "tailscale.toolchain.rev" {
+			return s.Value
+		}
+	}
+	return ""
+})
+
 func gitCommit() string {
 	if gitCommitStamp != "" {
 		return gitCommitStamp