[tailscale1.18] cmd/dist: stamp git commit into runtime.Version value

Temporary fix until golang#37475 is done.

(cherry picked from commit ba89304)

Change-Id: I204f03bc62c6f2295a8bc66d3f0bb2132153bc27

src/cmd/dist/build.go

@@ -377,6 +377,9 @@ func findgoversion() string {
 					b = "builder " + hostType
 				}
 			}
+			if isGitRepo() {
+				b += "-ts" + chomp(run(goroot, CheckExit, "git", "log", "-n", "1", "--format=format:%h", "HEAD"))
+			}
 			return b
 		}
 	}