cmd/go: add -d flag to mod_get_test

'go get all' was run in this test without -d. This caused some std
packages to be reinstalled if the test is run in a slightly different
configuration than make.bash was run. run.bash would fail in some
situations because of this. Nothing in the cmd/go tests should modify
installed std or cmd packages.

Updates #35459

Change-Id: Idd259a27d55502923b7fc54f361a77f0ac11eea2
Reviewed-on: https://go-review.googlesource.com/c/go/+/215721
Run-TryBot: Jay Conrod <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Michael Matloob <[email protected]>

Author: Jay Conrod

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

@@ -33,7 +33,7 @@ grep 'rsc.io/quote v1.5.1$' go.mod
 
 # 'go get all' should consider test dependencies with or without -t.
 cp go.mod.empty go.mod
-go get all
+go get -d all
 grep 'rsc.io/quote v1.5.2$' go.mod
 
 -- go.mod.empty --