cmd/go/internal/fips140: ignore GOEXPERIMENT on error

During toolchain selection, the GOEXPERIMENT value may not be valid for
the current version (but it is valid for the selected version). In this
case, cfg.ExperimentErr is set and cfg.Experiment is nil.

Normally cmd/go main exits when ExperimentErr is set, so Experiment is
~never nil. But that is skipped during toolchain selection, and
fips140.Init is used during toolchain selection.

Fixes #74111.

Change-Id: I6a6a636c65ee5831feaf3d29993a60613bbec6f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/680976
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>

Author: prattmic

src/cmd/go/internal/fips140/fips140.go

@@ -114,7 +114,11 @@ func Init() {
 		fsys.Bind(Dir(), filepath.Join(cfg.GOROOT, "src/crypto/internal/fips140"))
 	}
 
-	if cfg.Experiment.BoringCrypto && Enabled() {
+	// ExperimentErr != nil if GOEXPERIMENT failed to parse. Typically
+	// cmd/go main will exit in this case, but it is allowed during
+	// toolchain selection, as the GOEXPERIMENT may be valid for the
+	// selected toolchain version.
+	if cfg.ExperimentErr == nil && cfg.Experiment.BoringCrypto && Enabled() {
 		base.Fatalf("go: cannot use GOFIPS140 with GOEXPERIMENT=boringcrypto")
 	}
 }