Fix: Do not build exporter for linux/386

Building fails with a really bad exception:

```
=> ERROR [linux/386 builder 5/8] RUN go mod download                                                                                                         0.5s
------
 > [linux/386 builder 5/8] RUN go mod download:
```

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

@@ -47,8 +47,8 @@ runs:
         context: postfix_exporter
         push: true
         tags: '${{ inputs.tags }}'
-        platforms: "linux/386,linux/amd64,linux/arm/v5,linux/arm/v7,linux/arm64/v8,linux/mips64le,linux/ppc64le,linux/s390x"
-        # ,linux/arm/v6,,,,linux/arm64,,,,linux/riscv64"
+        platforms: "linux/amd64,linux/arm/v5,linux/arm/v7,linux/arm64/v8,linux/mips64le,linux/ppc64le,linux/s390x"
+        # linux/386,linux/arm/v6,linux/arm64,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