fix: use URL-safe base64 encoding

Prior to this commit, this used standard encoding, which worked up until
relatively recently. Now, unfortunately, using standard encoding appears
to be unsupported:

```
error: unexpected status code from GitHub API (401): {
  "message": "Bad credentials",
  "documentation_url": "https://docs.github.com/rest",
  "status": "401"
}
```

This is, in a sense, not surprising because JWTs are typically base64
URL-safe encoded. It's surprising that this worked before, however.

Author: ericnorris

internal/jwt.go

@@ -50,7 +50,7 @@ func (s *payloadSigner) appendPayload(payload any) (string, error) {
 		return "", err
 	}
 
-	return jwtHeader + "." + base64.StdEncoding.EncodeToString(jsonPayload), nil
+	return jwtHeader + "." + base64.RawURLEncoding.EncodeToString(jsonPayload), nil
 }
 
 func (s *payloadSigner) appendSignature(hp string) (string, error) {
@@ -62,5 +62,5 @@ func (s *payloadSigner) appendSignature(hp string) (string, error) {
 		return "", err
 	}
 
-	return hp + "." + base64.StdEncoding.EncodeToString(signature), nil
+	return hp + "." + base64.RawURLEncoding.EncodeToString(signature), nil
 }