cmd/dist: make fortran test conditional on bash existence

Fixes #14929.

Change-Id: I0391acf9f5f65389f73637533306a7c4240320b8
Reviewed-on: https://go-review.googlesource.com/21295Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/dist/test.go

@@ -445,7 +445,7 @@ func (t *tester) registerTests() {
 		if fortran == "" {
 			fortran, _ = exec.LookPath("gfortran")
 		}
-		if fortran != "" && t.goos != "dragonfly" { // see golang.org/issue/14544
+		if t.hasBash() && fortran != "" && t.goos != "dragonfly" { // see golang.org/issue/14544
 			t.tests = append(t.tests, distTest{
 				name:    "cgo_fortran",
 				heading: "../misc/cgo/fortran",