Signed-Releases: really look for *.sign files (#1298)

With this patch applied projects like dracut pass the check:
```
  "checks": [
    {
      "details": [
        "Debug: GitHub release found: 055",
        "Info: signed release artifact: dracut-055.tar.sign: https://api.github.com/repos/dracutdevs/dracut/releases/assets/37635937",
        "Debug: GitHub release found: 054",
        "Info: signed release artifact: dracut-054.tar.sign: https://api.github.com/repos/dracutdevs/dracut/releases/assets/36958052",
        "Debug: GitHub release found: 053",
        "Info: signed release artifact: dracut-053.tar.sign: https://api.github.com/repos/dracutdevs/dracut/releases/assets/32484038",
        "Debug: GitHub release found: 052",
        "Info: signed release artifact: dracut-052.tar.sign: https://api.github.com/repos/dracutdevs/dracut/releases/assets/32130796",
        "Debug: GitHub release found: 051",
        "Info: signed release artifact: dracut-051.tar.sign: https://api.github.com/repos/dracutdevs/dracut/releases/assets/31933850"
      ],
      "score": 10,
      "reason": "5 out of 5 artifacts are signed -- score normalized to 10",
      "name": "Signed-Releases",
```

checks/signed_releases.go

@@ -28,7 +28,7 @@ const (
 	releaseLookBack     = 5
 )
 
-var artifactExtensions = []string{".asc", ".minisig", ".sig"}
+var artifactExtensions = []string{".asc", ".minisig", ".sig", ".sign"}
 
 //nolint:gochecknoinits
 func init() {

docs/checks.md

@@ -495,7 +495,7 @@ Signed releases attest to the provenance of the artifact.
 
 This check looks for the following filenames in the project's last five
 releases: [*.minisig ](https://github.com/jedisct1/minisign), *.asc (pgp),
-*.sign.
+*.sig, *.sign.
 
 Note: The check does not verify the signatures. 
 

docs/checks/internal/checks.yaml

@@ -568,7 +568,7 @@ checks:
 
       This check looks for the following filenames in the project's last five
       releases: [*.minisig ](https://github.com/jedisct1/minisign), *.asc (pgp),
-      *.sign.
+      *.sig, *.sign.
 
       Note: The check does not verify the signatures. 
     remediation: