gopls/internal/telemetry/cmd/stacks: don't forward GOEXPERIMENT from env

The test being executed may have some GOEXPERIMENT set. When invoking
another go toolchain, since GOTOOLCHAIN is set to a different version
that might be older and not support the same GOEXPERIMENT values, let
it run with its own default experiments.

Fixes golang/go#71260.

Change-Id: Iadea59fbd8bf7a11e636208567ca0fd23cdb7fa1
Cq-Include-Trybots: luci.golang.try:x_tools-gotip-linux-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/tools/+/641859
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

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

@@ -1011,6 +1011,7 @@ func readPCLineTable(info Info, stacksDir string) (map[string]FileLine, error) {
 		cmd.Dir = buildDir
 		cmd.Env = append(os.Environ(),
 			"GOTOOLCHAIN="+info.GoVersion,
+			"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",
@@ -1026,6 +1027,7 @@ func readPCLineTable(info Info, stacksDir string) (map[string]FileLine, error) {
 	cmd.Stderr = os.Stderr
 	cmd.Env = append(os.Environ(),
 		"GOTOOLCHAIN="+info.GoVersion,
+		"GOEXPERIMENT=", // Don't forward GOEXPERIMENT from current environment since the GOTOOLCHAIN selected might not support the same experiments.
 		"GOOS="+info.GOOS,
 		"GOARCH="+info.GOARCH,
 	)