chore(linux): Fix coverage report creation for keyman-system-service

Since keyman-system-service is a dependency of ibus-keyman it gets
build when ibus-keyman gets build. However, in that case the `--coverage`
flag doesn't get passed through in which case the `coverage-html`
target is missing from ninja. This change fixes this problem by checking
if ninja has the `coverage-html` target. If not we remove the output
directory which causes the configure action to run again with the
`--coverage` option.