Fix: disable building postfix-expoter for ppc64le and s390x

Builds fail:

```
[linux/s390x builder 8/8] RUN env GOOS="linux" GOARCH="s390x" GOARM="$( echo "" | grep -E -o "\d+$")" go build -ldflags '-extldflags "-static"'  -o /bin/postfix_exporter
606.1 net: gcc: signal: segmentation fault (core dumped)
...
```

```
[linux/ppc64le builder 8/8] RUN env GOOS="linux" GOARCH="ppc64le" GOARM="$( echo "" | grep -E -o "\d+$")" go build -ldflags '-extldflags "-static"'  -o /bin/postfix_exporter
ERROR: process "/bin/sh -c env GOOS=\"${TARGETOS}\" GOARCH=\"${TARGETARCH}\" GOARM=\"$( echo \"$TARGETVARIANT\" | grep -E -o \"\\\\d+$\")\" go build -ldflags '-extldflags \"-static\"'  -o /bin/postfix_exporter" did not complete successfully: exit code: 1
gcc: internal compiler error: Segmentation fault signal terminated program cc1
Please submit a full bug report, with preprocessed source (by using -freport-bug).
See <file:///usr/share/doc/gcc-12/README.Bugs> for instructions.
```

Author: bokysan

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

@@ -68,7 +68,8 @@ runs:
         push: true
         tags: '${{ inputs.tags }}'
         labels: ${{ steps.meta.outputs.labels }}
-        platforms: "linux/amd64,linux/arm/v5,linux/arm/v7,linux/arm64/v8,linux/mips64le,linux/ppc64le,linux/s390x"
+        platforms: "linux/amd64,linux/arm/v5,linux/arm/v7,linux/arm64/v8,linux/mips64le"
+        #platforms: "linux/amd64,linux/arm/v5,linux/arm/v7,linux/arm64/v8,linux/mips64le,linux/ppc64le,linux/s390x"
         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