Fix: Remove build for linux/mips64le and linux/arm/v5 for postfix-exp…

…orter

There's some weird bug where building the exporter for `linux/mips64le`
and `linux/arm/v5` has go complaining how
(go-systemd)[https://github.com/coreos/go-systemd/tree/main] has no
linux packages.

We're taking a quick-and-ditry route here and hope that nobody's going
to use the postfix-exporter on MIPS / ARMv5, which hopefully won't be such a big
issue.

.github/actions/build-postfix-exporter/action.yaml

@@ -47,7 +47,7 @@ runs:
         context: postfix_exporter
         push: true
         tags: '${{ inputs.tags }}'
-        platforms: "linux/386,linux/arm/v5,linux/arm/v6,linux/arm/v7,linux/arm64/v8,linux/amd64,linux/arm64,linux/ppc64le,linux/s390x,linux/mips64le,linux/riscv64"
+        platforms: "linux/386,linux/arm/v6,linux/arm/v7,linux/arm64/v8,linux/amd64,linux/arm64,linux/ppc64le,linux/s390x,linux/riscv64"
         cache-from: type=local,src=/tmp/.buildx-cache/postfix-exporter,mode=max,compression=estargz
         cache-to: type=local,dest=/tmp/.buildx-cache-new/postfix-exporter