Disable binary signing in snapshot mode

Snapshot builds failed for external contributors due to cosign.
Since snapshot builds are not official releases, we do not need
to sign binaries. Disabling signing in this mode fixes the issue.

Signed-off-by: Bryan Frimin <bryan@getprobo.com>

.goreleaser.yaml

@@ -74,16 +74,18 @@ sboms:
       - "--source-version={{ .Version }}"
 
 signs:
-  - cmd: cosign
+  - cmd: sh
     env:
       - COSIGN_EXPERIMENTAL=1
     certificate: "${artifact}.pem"
     args:
-      - sign-blob
-      - "--output-certificate=${certificate}"
-      - "--output-signature=${signature}"
-      - "${artifact}"
-      - "--yes"
+      - -c
+      - |
+        if [ "{{ .IsSnapshot }}" = "true" ]; then
+          echo "Skipping signing (snapshot: {{ .IsSnapshot }})"
+        else
+          cosign sign-blob --output-certificate="${certificate}" --output-signature="${signature}" "${artifact}" --yes
+        fi
     artifacts: checksum
     output: true
 
@@ -101,25 +103,33 @@ docker_manifests:
 
 docker_signs:
   - id: images
-    cmd: cosign
+    cmd: sh
     env:
       - COSIGN_EXPERIMENTAL=1
     artifacts: images
     output: true
     args:
-      - "sign"
-      - "${artifact}@${digest}"
-      - "--yes"
+      - -c
+      - |
+        if [ "{{ .IsSnapshot }}" = "true" ]; then
+          echo "Skipping Docker image signing (snapshot: {{ .IsSnapshot }})"
+        else
+          cosign sign "${artifact}@${digest}" --yes
+        fi
   - id: manifests
-    cmd: cosign
+    cmd: sh
     env:
       - COSIGN_EXPERIMENTAL=1
     artifacts: manifests
     output: true
     args:
-      - "sign"
-      - "${artifact}@${digest}"
-      - "--yes"
+      - -c
+      - |
+        if [ "{{ .IsSnapshot }}" = "true" ]; then
+          echo "Skipping Docker manifest signing (snapshot: {{ .IsSnapshot }})"
+        else
+          cosign sign "${artifact}@${digest}" --yes
+        fi
 
 dockers:
   - image_templates: