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.

crossbuild.bash

@@ -1,4 +1,6 @@
 #!/bin/bash -eu
+#
+# Build on all supported architectures & operating systems
 
 cd "$(dirname "$0")"
 
@@ -7,6 +9,10 @@ B="go build -tags without_openssl"
 
 set -x
 
+export CGO_ENABLED=0
+
+GOOS=linux  GOARCH=amd64         $B
+
 # See https://github.com/golang/go/wiki/GoArm
 GOOS=linux  GOARCH=arm   GOARM=7 $B
 GOOS=linux  GOARCH=arm64         $B