Pin DeterminateSystems/nix-installer-action to SHA (#32)

GitHub recommends pinning 3rd party actions for security and
reproducibility. This should be a familiar idea for folks who've worked
with Nix. 😄


https://docs.github.com/en/actions/security-guides/security-hardening-for-github-actions#using-third-party-actions

For now, I've excluded dependencies from `actions/` as these are "2nd
party" actions, developed and published by GitHub. We have higher trust
that GitHub published actions won't spuriously force push tag values,
resulting in non deterministic runtime behavior.

If you'd like to pin all dependencies, let me know and I'll update this
to include everything. This will result in a higher sustaining burden,
as Dependabot
(#31) will
generate many more updates for pinned `actions/` actions.

action.yml

@@ -127,7 +127,7 @@ runs:
 
     - name: Install nix
       if: inputs.skip-nix-installation == 'false'
-      uses: DeterminateSystems/nix-installer-action@v4
+      uses: DeterminateSystems/nix-installer-action@65d7c888b2778e8cf30a07a88422ccb23499bfb8  # v4
       with:
         logger: pretty
         extra-conf: experimental-features = ca-derivations fetch-closure