.github: update and pin actions/cache to latest 4.x (tailscale#13555)

Update and pin actions/cache usage to latest 4.x. These were previously
pointing to `@3` which pulls in the latest v3 as they are released, with
the potential to break our workflows if a breaking change or malicious
version on the `@3` stream is ever pushed.

Changing this to a pinned version also means that dependabot will keep
this in the pinned version format (e.g., referencing a SHA) when it
opens a PR to bump the dependency.

The breaking change between v3 and v4 is that v4 requires Node 20 which
should be a non-issue where this is run.

Updates #cleanup

Signed-off-by: Mario Minardi <mario@tailscale.com>

.github/workflows/test.yml

@@ -80,7 +80,7 @@ jobs:
     - name: checkout
       uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
     - name: Restore Cache
-      uses: actions/cache@v3
+      uses: actions/cache@0c45773b623bea8c8e75f6c82b208c3cf94ea4f9 # v4.0.2
       with:
         # Note: unlike the other setups, this is only grabbing the mod download
         # cache, rather than the whole mod directory, as the download cache
@@ -159,7 +159,7 @@ jobs:
         cache: false
 
     - name: Restore Cache
-      uses: actions/cache@v3
+      uses: actions/cache@0c45773b623bea8c8e75f6c82b208c3cf94ea4f9 # v4.0.2
       with:
         # Note: unlike the other setups, this is only grabbing the mod download
         # cache, rather than the whole mod directory, as the download cache
@@ -260,7 +260,7 @@ jobs:
     - name: checkout
       uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
     - name: Restore Cache
-      uses: actions/cache@v3
+      uses: actions/cache@0c45773b623bea8c8e75f6c82b208c3cf94ea4f9 # v4.0.2
       with:
         # Note: unlike the other setups, this is only grabbing the mod download
         # cache, rather than the whole mod directory, as the download cache
@@ -319,7 +319,7 @@ jobs:
     - name: checkout
       uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
     - name: Restore Cache
-      uses: actions/cache@v3
+      uses: actions/cache@0c45773b623bea8c8e75f6c82b208c3cf94ea4f9 # v4.0.2
       with:
         # Note: unlike the other setups, this is only grabbing the mod download
         # cache, rather than the whole mod directory, as the download cache
@@ -367,7 +367,7 @@ jobs:
     - name: checkout
       uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
     - name: Restore Cache
-      uses: actions/cache@v3
+      uses: actions/cache@0c45773b623bea8c8e75f6c82b208c3cf94ea4f9 # v4.0.2
       with:
         # Note: unlike the other setups, this is only grabbing the mod download
         # cache, rather than the whole mod directory, as the download cache