coverage.yml and main.yml: silence actionlint warnings

[cooljeanius]

Without this PR, actionlint prints the following warnings on this repo's GitHub Actions workflows:

$ actionlint
.github/workflows/coverage.yml:33:9: shellcheck reported issue in this script: SC2046:warning:2:8: Quote this to prevent word splitting [shellcheck]
   |
33 |         run: |
   |         ^~~~
.github/workflows/coverage.yml:45:9: shellcheck reported issue in this script: SC2086:info:5:119: Double quote to prevent globbing and word splitting [shellcheck]
   |
45 |         run: |
   |         ^~~~
.github/workflows/coverage.yml:45:9: shellcheck reported issue in this script: SC2086:info:6:84: Double quote to prevent globbing and word splitting [shellcheck]
   |
45 |         run: |
   |         ^~~~
.github/workflows/main.yml:28:9: shellcheck reported issue in this script: SC2006:style:1:7: Use $(...) notation instead of legacy backticks `...` [shellcheck]
   |
28 |         run: |
   |         ^~~~
.github/workflows/main.yml:46:9: shellcheck reported issue in this script: SC2046:warning:4:18: Quote this to prevent word splitting [shellcheck]
   |
46 |         run: |
   |         ^~~~
.github/workflows/main.yml:46:9: shellcheck reported issue in this script: SC2046:warning:6:17: Quote this to prevent word splitting [shellcheck]
   |
46 |         run: |
   |         ^~~~

This PR silences them.

[cooljeanius]

Would it make sense for me to handle yamllint warnings in this PR, too, or should I open a separate PR for that?

[ryandesign]

There's no need to add braces here.

[cooljeanius]

it helps me grok more easily where exactly the variable begins and ends