Only fuzz test PRs that run the test suite

This adds a `paths` restriction on the CIFuzz job to avoid the
situation where a PR that doesn't modify anything that causes the
usual tests to run on it would still run fuzz tests.

(This should still have CIFuzz run even when the only change a PR
makes is to adjust how CI fuzzing itself works, because one of the
patterns is matched by any change to any CI workflow file.)

.github/workflows/cifuzz.yml

@@ -1,7 +1,18 @@
 name: CIFuzz
 on:
   pull_request:
-    branches: [ main ]
+    branches:
+      - main
+    paths:
+      - '.github/**'
+      - 'ci/**'
+      - 'etc/**'
+      - 'src/**'
+      - 'tests/**'
+      - 'cargo-*/**'
+      - 'gix*/**'
+      - '*.toml'
+      - Makefile
   workflow_dispatch:
 jobs:
  Fuzzing: