ci: validate actions and fix actionlint warnings

ci: validate actions and fix actionlint warnings

some actions like Coverity and CFLite aren't run on every PR so to make
sure they are more or less fine when they are changed it makes sense to
at least check them with superlinter/actionlint: https://github.com/rhysd/actionlint

The following warnings were fixed along the way:
```
.github/workflows/mkosi.yml:55:7: shellcheck reported issue in this script: SC2086:info:6:14: Double quote to prevent globbing and word splitting [shellcheck]
   |
55 |       run: |
   |       ^~~~
.github/workflows/mkosi.yml:55:7: shellcheck reported issue in this script: SC2046:warning:6:40: Quote this to prevent word splitting [shellcheck]
   |
55 |       run: |
   |       ^~~~
.github/workflows/mkosi.yml:55:7: shellcheck reported issue in this script: SC2006:style:6:40: Use $(...) notation instead of legacy backticked `...` [shellcheck]
   |
55 |       run: |
   |       ^~~~
```
```
.github/workflows/coverity.yml:31:9: shellcheck reported issue in this script: SC2086:info:1:93: Double quote to prevent globbing and word splitting [shellcheck]
   |
31 |         run: echo "COVERITY_SCAN_NOTIFICATION_EMAIL=$(git log -1 ${{ github.sha }} --pretty=\"%aE\")" >> $GITHUB_ENV
   |         ^~~~
```