fix(ci): update secret-scanner SHA to @7fdc2705df74b4e352d2a1cde3e87a5923fdf329 + simplify permissions (#44)

Update secret-scanner-reusable.yml pin to
@7fdc2705df74b4e352d2a1cde3e87a5923fdf329. Remove excessive job-level
permissions. Remove erroneous squisher-corpus guix.scm placeholders.

Part of estate-wide hyperpolymath/standards#426 remediation.

Author: hyperpolymath

.github/workflows/secret-scanner.yml

@@ -17,7 +17,5 @@ jobs:
   scan:
     permissions:
       contents: read
-      pull-requests: write
-      actions: read
-    uses: hyperpolymath/standards/.github/workflows/secret-scanner-reusable.yml@d135b05bfc647d0c0fbfedc7e80f37ea50f49236
+    uses: hyperpolymath/standards/.github/workflows/secret-scanner-reusable.yml@7fdc2705df74b4e352d2a1cde3e87a5923fdf329
     secrets: inherit