Use pinned Linux version

We really shouldn't be using the moving target that is "the latest"
Linux kernel version: just changing a config flag would introduce a
bunch of completely unrelated changes, including potentially
wrecking a config if certain high profile options were to be renamed.

Fixate the Linux kernel version so that we have full control over what
we use. Stick to 6.18, which is what commit 991dd4b ("Update
vmlinux.h headers from Linux 6.14 to 6.18") bumped us to.

Signed-off-by: Daniel Müller <deso@posteo.net>

Author: d-e-s-o

.github/workflows/vmlinux.h.yml

@@ -16,9 +16,13 @@ jobs:
 
       - uses: actions/checkout@v4
 
-      - name: Download Linux source
-        shell: bash
-        run: ./scripts/download-latest-linux-release.sh
+      - name: Check out Linux source
+        uses: actions/checkout@v6
+        with:
+          repository: 'torvalds/linux'
+          ref: 'v6.18'
+          fetch-depth: 1
+          path: linux/
 
       - name: Install dependencies
         shell: bash