test: enable parallel pre-commit shellcheck

SC1090 appears to be the only thing requiring serial runs, but we have
it disabled anyway.

.pre-commit-config.yaml

@@ -38,6 +38,7 @@ repos:
         types: [text]
         files: ^(bash_completion|completions/.+|test/run*|.+\.sh(\.in)?)$
         exclude: completions/(\.gitignore|Makefile.*)$
+        require_serial: false  # We disable SC1090 anyway, so parallel is ok
   - repo: https://github.com/perltidy/perltidy
     rev: cbb4a687dd331db69219599d84d527397402efdd
     hooks: