cmd/go: clear GIT_ALLOW_PROTOCOL during tests

Clear it before any tests begin.
Clear it again after TestIsSecureGitAllowProtocol sets it.

Fixes #17700.

Change-Id: I6ea50d37f8222d8c7c9fee0b1eac3bbdfb5d133e
Reviewed-on: https://go-review.googlesource.com/32640Reviewed-by: Quentin Smith <quentin@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>

src/cmd/go/go_test.go

@@ -99,6 +99,7 @@ func TestMain(m *testing.M) {
 	// Don't let these environment variables confuse the test.
 	os.Unsetenv("GOBIN")
 	os.Unsetenv("GOPATH")
+	os.Unsetenv("GIT_ALLOW_PROTOCOL")
 	if home, ccacheDir := os.Getenv("HOME"), os.Getenv("CCACHE_DIR"); home != "" && ccacheDir == "" {
 		// On some systems the default C compiler is ccache.
 		// Setting HOME to a non-existent directory will break

src/cmd/go/vcs_test.go

@@ -271,6 +271,7 @@ func TestIsSecureGitAllowProtocol(t *testing.T) {
 		{vcsBzr, "foo://example.com/bar.bzr", false},
 	}
 
+	defer os.Unsetenv("GIT_ALLOW_PROTOCOL")
 	os.Setenv("GIT_ALLOW_PROTOCOL", "https:foo")
 	for _, test := range tests {
 		secure := test.vcs.isSecure(test.url)