cmd/go: do not call base.fatal for an unset HOME for GOAUTH=netrc

This CL silences errors caused by GOAUTH=netrc and HOME being unset.
Instead, we log the error if the -x flag is set.

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-windows-amd64-longtest
Change-Id: Ibd323769f3562c169ebf559e060e9afbb63047bc
Reviewed-on: https://go-review.googlesource.com/c/go/+/643917
Reviewed-by: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/go/internal/auth/auth.go

@@ -70,7 +70,8 @@ func runGoAuth(client *http.Client, res *http.Response, url string) {
 		case "netrc":
 			lines, err := readNetrc()
 			if err != nil {
-				base.Fatalf("go: could not parse netrc (GOAUTH=%s): %v", cfg.GOAUTH, err)
+				cmdErrs = append(cmdErrs, fmt.Errorf("GOAUTH=%s: %v", command, err))
+				continue
 			}
 			// Process lines in reverse so that if the same machine is listed
 			// multiple times, we end up saving the earlier one

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

@@ -53,6 +53,19 @@ go get vcs-test.golang.org/auth/or401
 env NETRC=$WORK/missing
 ! go get vcs-test.golang.org/auth/or401
 stderr '^\tserver response: ACCESS DENIED, buddy$'
+
+[short] skip 'requires a remote vcs lookup'
+[!git] skip
+# An unset home directory should warn the user but not cause a failure.
+env NETRC=
+env HOME=
+env USERPROFILE=
+env home=
+go get -x vcs-test.golang.org/git/emptytest.git
+[!GOOS:windows] [!GOOS:plan9] stderr 'GOAUTH=netrc: \$HOME is not defined'
+[GOOS:windows] stderr 'GOAUTH=netrc: \%userprofile\% is not defined'
+[GOOS:plan9] stderr 'GOAUTH=netrc: \$home is not defined'
+
 -- go.mod --
 module private.example.com
 -- $WORK/empty --