ci: Pin runner in hlint, nix workflows to ubuntu-20.04

Both are now broken with ubuntu-22.04. Nix:

    error: could not set permissions on '/nix/var/nix/profiles/per-user' to 755: Operation not permitted

hlint:

    /opt/hostedtoolcache/hlint/3.4.1/x64/hlint: error while loading shared libraries: libtinfo.so.5: cannot open shared object file: No such file or directory

.github/workflows/hlint.yaml

@@ -6,7 +6,7 @@ on:
 
 jobs:
   hlint:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04  # FIXME
     steps:
     - uses: actions/checkout@v3
 

.github/workflows/nix.yml

@@ -6,7 +6,7 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04  # FIXME
     name: Nix Flake - Linux
     permissions:
       contents: read