gopls/internal/telemetry/cmd/stacks: build compiler from root

Currently we build cmd/compile from the current directory.
Unfortunately, if the current directory happens to contain a go.mod
file, the `go` directive restricts the GOTOOLCHAINs that are allowed.

For example, gopls go.mod says `go 1.23.4`. Running stacks from its
directory is thus unable to build a compiler older than 1.23.4.

To avoid this, switch to root when building the compiler, assuming that
root won't contain a go.mod.

We can now drop GOWORK=off, as root presumably won't contain a go.work
either.

Change-Id: I6a6a636ccb6fd2b04db3352e17dc28310fc8d069
Reviewed-on: https://go-review.googlesource.com/c/tools/+/642935
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

Author: prattmic

gopls/internal/telemetry/cmd/stacks/stacks.go

@@ -989,6 +989,11 @@ func readPCLineTable(info Info, stacksDir string) (map[string]FileLine, error) {
 		buildDir = filepath.Join(revDir, "gopls")
 	case "cmd/compile":
 		// Nothing to do, GOTOOLCHAIN is sufficient.
+
+		// Switch build directories so if we happen to be in Go module
+		// directory its go.mod doesn't restrict the toolchain versions
+		// we're allowed to use.
+		buildDir = "/"
 	default:
 		return nil, fmt.Errorf("don't know how to build unknown program %s", info.Program)
 	}
@@ -1014,7 +1019,6 @@ func readPCLineTable(info Info, stacksDir string) (map[string]FileLine, error) {
 			"GOEXPERIMENT=", // Don't forward GOEXPERIMENT from current environment since the GOTOOLCHAIN selected might not support the same experiments.
 			"GOOS="+info.GOOS,
 			"GOARCH="+info.GOARCH,
-			"GOWORK=off",
 		)
 		if err := cmd.Run(); err != nil {
 			return nil, fmt.Errorf("building: %v (rm -fr %s?)", err, stacksDir)