fix(build): Use clean build for coverage test run

[HenningHolmDE]

Previously, coverage data could be messed up by build artifacts restored from the GitHub cache. While it is not yet cleared up how this actually happens, removing all build data before executing the coverage test run seems to resolve this issue.

This fixes #508.

[coveralls]

Pull Request Test Coverage Report for Build 9568074175

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+2.4%) to 92.324%

---

Totals
Change from base Build 9539427042:	2.4%
Covered Lines:	9959
Relevant Lines:	10787

---

💛 - Coveralls

[HenningHolmDE]

Yay, +2.4% coverage increase just by cleaning the build directory. 😄

[duesee]

Thanks, happy to merge (and curiously looking forward to the rebase :-))