cmd/go: respect GODEBUG in test cache

GODEBUG affects test execution but was not being tracked.

Fixes #66213.
Fixes #65436.

Change-Id: I3ac3c397f0c6fa46cd9be0d22d03020d0632f64f
Reviewed-on: https://go-review.googlesource.com/c/go/+/570259
Reviewed-by: Ian Lance Taylor <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Russ Cox <[email protected]>

Author: rsc

src/cmd/go/internal/test/test.go

@@ -1859,6 +1859,8 @@ var testlogMagic = []byte("# test log\n") // known to testing/internal/testdeps/
 func computeTestInputsID(a *work.Action, testlog []byte) (cache.ActionID, error) {
 	testlog = bytes.TrimPrefix(testlog, testlogMagic)
 	h := cache.NewHash("testInputs")
+	// The runtime always looks at GODEBUG, without telling us in the testlog.
+	fmt.Fprintf(h, "env GODEBUG %x\n", hashGetenv("GODEBUG"))
 	pwd := a.Package.Dir
 	for _, line := range bytes.Split(testlog, []byte("\n")) {
 		if len(line) == 0 {

src/cmd/go/testdata/script/test_cache_inputs.txt

@@ -23,6 +23,14 @@ go test testcache -run=TestLookupEnv
 go test testcache -run=TestLookupEnv
 stdout '\(cached\)'
 
+# GODEBUG is always read
+env GODEBUG=asdf=1
+go test testcache -run=TestLookupEnv
+! stdout '\(cached\)'
+go test testcache -run=TestLookupEnv
+stdout '\(cached\)'
+env GODEBUG=
+
 env TESTKEY=y
 go test testcache -run=TestLookupEnv
 ! stdout '\(cached\)'