workflows: switch to new default branch main

.github/workflows/check.yml

@@ -2,7 +2,7 @@ name: "Check & Cachix"
 on:
   push:
     branches:
-      - master
+      - main
       - trying
       - staging
 jobs:

.github/workflows/mdbook_docs.yml

@@ -3,7 +3,7 @@ name: Deploy Docs to GitHub Pages
 on:
   push:
     branches:
-      - master
+      - main
 
 jobs:
   deploy:

.github/workflows/release.yml

@@ -19,7 +19,7 @@ jobs:
       - name: Checkout code
         uses: actions/checkout@v2
         with:
-          ref: master
+          ref: main
       - name: Update Changelog
         uses: heinrichreimer/github-changelog-generator-action@v2.1.1
         with:
@@ -49,7 +49,7 @@ jobs:
       - name: Checkout code
         uses: actions/checkout@v2
         with:
-          ref: master
+          ref: main
 
       - name: Get Changelog Entry
         id: changelog_reader