cmd/dist: disable FIPS 140-3 mode when testing maphash with purego

hash/maphash can't be built with FIPS 140-3 mode and the purego tag
since CL 703095. That change precludes running "GODEBUG=fips140=on go
dist test" with, as there is a test variant that tests maphash with the
purego tag.

Change-Id: Iaedfaf3bb79281a799ef95283310c96d2e64207f
Reviewed-on: https://go-review.googlesource.com/c/go/+/705775
Auto-Submit: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

Author: qmuntal

src/cmd/dist/test.go

@@ -705,6 +705,7 @@ func (t *tester) registerTests() {
 				timeout: 300 * time.Second,
 				tags:    []string{"purego"},
 				pkg:     "hash/maphash",
+				env:     []string{"GODEBUG=fips140=off"}, // FIPS 140-3 mode is incompatible with purego
 			})
 	}