prow.sh: enable building arm64 image

prow.sh

@@ -85,7 +85,7 @@ get_versioned_variable () {
     echo "$value"
 }
 
-configvar CSI_PROW_BUILD_PLATFORMS "linux amd64; windows amd64 .exe; linux ppc64le -ppc64le; linux s390x -s390x" "Go target platforms (= GOOS + GOARCH) and file suffix of the resulting binaries"
+configvar CSI_PROW_BUILD_PLATFORMS "linux amd64; windows amd64 .exe; linux ppc64le -ppc64le; linux s390x -s390x; linux arm64 -arm64" "Go target platforms (= GOOS + GOARCH) and file suffix of the resulting binaries"
 
 # If we have a vendor directory, then use it. We must be careful to only
 # use this for "make" invocations inside the project's repo itself because