Build multiarch image in test-docker.bash (#25939)

Build `linux/amd64` platform image in addition to system default
`linux/arm64` in the nightly Docker test script.

In addition to improving test coverage, this will cause our `nightly`
image tag to be multi-arch going forward.

To be merged after
https://github.hpe.com/hpe/hpc-chapel-ci-config/pull/1280 adjusts this
job to run on our test machine that supports multi-arch build.

Implements Cray/chapel-private#6563.

[reviewed by @tzinsky , thanks!]

Testing:
- [x] manual run on M1 testing mac

util/cron/test-docker.bash

@@ -15,7 +15,7 @@ build_image() {
   # Remove any existing image with the tag before building docker image
   docker image rm --force $imageName
 
-  docker build --push . -t $imageName
+  docker buildx build --platform=linux/amd64,linux/arm64 . --push -t $imageName
   BUILD_RESULT=$?
   if [ $BUILD_RESULT -ne 0 ]
   then