BLD: Rework to use newer cachix

Author: HaoZeke

.github/workflows/build_apidocs.yml

@@ -17,7 +17,7 @@ jobs:
     - uses: cachix/install-nix-action@v12
       with:
         nix_path: nixpkgs=channel:nixos-unstable
-    - uses: cachix/cachix-action@v8
+    - uses: cachix/cachix-action@v10
       with:
         name: dseams
     - run: nix-shell --run "doxygen Doxyfile-prj.cfg"

.github/workflows/build_pkg.yml

@@ -5,10 +5,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2.3.4
-    - uses: cachix/install-nix-action@v12
+    - uses: cachix/install-nix-action@v16
       with:
         nix_path: nixpkgs=channel:nixos-unstable
-    - uses: cachix/cachix-action@v8
+    - uses: cachix/cachix-action@v10
       with:
         name: dseams
         signingKey: '${{ secrets.CACHIX_SIGNING_KEY }}'