[github] Use macos-12

It might be beneficial to set up a matrix
([docs](https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#example-using-a-multi-dimension-matrix))
and run all major supported macOS versions in parallel.

Reason: macos-10.15 is [being deprecated](https://github.com/actions/runner-images/issues/5583) and
will be fully unsupported by Aug 30 2022.

.github/workflows/build.yml

@@ -10,7 +10,7 @@ on:
         - build
 jobs:
   build:
-    runs-on: macos-10.15
+    runs-on: macos-12
     steps:
     - uses: actions/checkout@v2
     - uses: cachix/install-nix-action@v9

.github/workflows/debug.yml

@@ -10,7 +10,7 @@ on:
         - debug
 jobs:
   debug:
-    runs-on: macos-10.15
+    runs-on: macos-12
     steps:
     - uses: actions/checkout@v2
     - uses: cachix/install-nix-action@v15

.github/workflows/test.yml

@@ -4,7 +4,7 @@ on:
   push:
 jobs:
   tests:
-    runs-on: macos-10.15
+    runs-on: macos-12
     timeout-minutes: 30
     steps:
     - uses: actions/checkout@v2
@@ -13,7 +13,7 @@ jobs:
     - run: nix-build ./release.nix -I nixpkgs=channel:nixpkgs-21.05-darwin -I darwin=. -A manpages
     - run: nix-build ./release.nix -I nixpkgs=channel:nixpkgs-21.05-darwin -I darwin=. -A examples.simple
   install:
-    runs-on: macos-10.15
+    runs-on: macos-12
     timeout-minutes: 30
     steps:
     - uses: actions/checkout@v2
@@ -39,7 +39,7 @@ jobs:
       with:
         limit-access-to-actor: true
   install-flake:
-    runs-on: macos-10.15
+    runs-on: macos-12
     timeout-minutes: 60
     steps:
     - uses: actions/checkout@v2

.github/workflows/update-manual.yml

@@ -8,7 +8,7 @@ on:
 
 jobs:
   update-manual:
-    runs-on: macos-10.15
+    runs-on: macos-12
     steps:
     - name: Checkout repository
       uses: actions/checkout@v3