crossbuild.bash: disable CGO
crossbuild.bash: disable CGO

build-without-openssl.bash also disables CGO, so
this makes it more real-world-y.

But the real reason is that disabling CGO hopefully
fixes this travis ci build failure:

+GOOS=darwin
+GOARCH=arm64
+go build -tags without_openssl
/home/travis/.gimme/versions/go1.13.15.linux.amd64/pkg/tool/linux_amd64/link: running gcc failed: exit status 1
/usr/bin/ld: unrecognized option '-pagezero_size'
/usr/bin/ld: use the --help option for usage information
collect2: error: ld returned 1 exit status
The command "./crossbuild.bash" exited with 2.

rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs