support Go versions with X: suffixes for GOEXPERIMENTs

A workaround until https://github.com/golang/go/issues/75953 is fixed.

See #978.

main.go

@@ -731,6 +731,9 @@ To install Go, see: https://go.dev/doc/install
 	if err := json.Unmarshal(out, &sharedCache.GoEnv); err != nil {
 		return fmt.Errorf(`cannot unmarshal from "go env -json": %w`, err)
 	}
+	// TODO: remove once https://github.com/golang/go/issues/75953 is fixed,
+	// such that `go env GOVERSION` can always be parsed by go/version.
+	sharedCache.GoEnv.GOVERSION, _, _ = strings.Cut(sharedCache.GoEnv.GOVERSION, " ")
 
 	// Some Go version managers switch between Go versions via a GOROOT which symlinks
 	// to one of the available versions. Given that later we build a patched linker

testdata/script/goversion.txtar

@@ -12,7 +12,7 @@ stderr 'Go version is too old; please upgrade to go1\.25\.0 or newer'
 # A clearly invalid go version.
 env TOOLCHAIN_GOVERSION='bogus version'
 ! exec garble build
-stderr 'Go version "bogus version" appears to be invalid or too old; use go1\.25\.0 or newer'
+stderr 'Go version "bogus" appears to be invalid or too old; use go1\.25\.0 or newer'
 
 # We should error on a devel version that's too old;
 # note that they started with the string "devel",
@@ -49,6 +49,12 @@ env TOOLCHAIN_GOVERSION='go1.25.0-somecustomversion'
 ! exec garble build
 stderr 'mocking the real build'
 
+# A stable version with a build-time GOEXPERIMENT.
+# See: https://github.com/golang/go/issues/75953
+env TOOLCHAIN_GOVERSION='go1.25.0 X:nodwarf5'
+! exec garble build
+stderr 'mocking the real build'
+
 # The current toolchain may be older than the one that built garble.
 env GARBLE_TEST_GOVERSION='go1.25.1'
 env TOOLCHAIN_GOVERSION='go1.25.0'