update gh workflows

because microsoft disabled some commands used in older nix-action and
broke the builds

.github/workflows/latexmk.yaml

@@ -7,13 +7,14 @@ jobs:
     latexmk:
         runs-on: ubuntu-latest
         steps:
-        - uses: actions/checkout@v2
+        - uses: actions/checkout@v2.3.4
           with:
               # Nix Flakes doesn't work on shallow clones
               fetch-depth: 0
-        - uses: cachix/install-nix-action@v11
+        - uses: cachix/install-nix-action@v13
           with:
             install_url: https://github.com/numtide/nix-flakes-installer/releases/download/nix-3.0pre20201007_5257a25/install
+            nix_path: nixpkgs=channel:nixos-unstable
             extra_nix_config: |
               experimental-features = nix-command flakes
         - run: nix build