ci: target the main branch instead of mainline

The workflows referenced a mainline branch that does not exist in this
repository; the default branch is main.

Basic CI and CodeQL are gated on `branches: [ mainline ]`, so they never
triggered for pull requests targeting main. The coverage check runs on all
branches, but failed at `git checkout origin/mainline` before it could
measure anything.

Signed-off-by: Ben Copeland <ben.copeland@linaro.org>

Author: bhcopeland

.github/workflows/basic-ci.yml

@@ -2,9 +2,9 @@ name: Basic CI
 
 on:
   push:
-    branches: [ mainline ]
+    branches: [ main ]
   pull_request:
-    branches: [ mainline ]
+    branches: [ main ]
 
 permissions:
   contents: read

.github/workflows/codeql.yml

@@ -2,9 +2,9 @@ name: "CodeQL"
 
 on:
   push:
-    branches: [ mainline ]
+    branches: [ main ]
   pull_request:
-    branches: [ mainline ]
+    branches: [ main ]
 
 jobs:
   analyze:

.github/workflows/coverage-check.yml

@@ -29,7 +29,7 @@ jobs:
 
     - name: Get base branch coverage
       run: |
-        git checkout origin/mainline
+        git checkout origin/main
         pip install -e ".[dev]"
         python -m pytest tests/ -m "not integration" --cov=src --cov-report=json:coverage-base.json --tb=no -q || true
         BASE_COVERAGE=$(python -c "import json; print(json.load(open('coverage-base.json'))['totals']['percent_covered'])")