Fix aarch64 action run (#113)

It seems there's a typo in the ARM64 case. 

The `desired_cosign_filename` variable is set to `_amd64`. This small patch fixes it.

Signed-off-by: Anastassios Nanos <ananos@nubificus.co.uk>

action.yml

@@ -105,7 +105,7 @@ runs:
               ARM64)
                 bootstrap_filename='cosign-linux-arm64'
                 bootstrap_sha=${bootstrap_linux_arm64_sha}
-                desired_cosign_filename='cosign-linux-amd64'
+                desired_cosign_filename='cosign-linux-arm64'
                 if [[ ${{ inputs.cosign-release }} == 'v0.6.0' ]]; then
                   log_error "linux-arm64 build not available at v0.6.0"
                   exit 1