Avoid workflow conditional reliance on specific runner

In order to catch platform-specific bugs, the "Test Go" workflow uses a job matrix to run the tests on multiple runners.
The step that uploads code coverage data to Codecov is intended to run only during the Linux job. The runner name
`ubuntu-latest` was used in the conditional to accomplish this. However, it might become necessary or desirable to pin a
specific runner version (e.g., `ubuntu-18.04`). The accompanying adjustment to the conditional might be forgotten and
there would not be any obvious sign that the coverage upload had stopped, nor why.

This will be avoided by using the general `runner.os` context item to identify the Linux job in the conditional. This
will not be ideal in the event multiple Linux runners are added to the workflow's job matrix, but this is less likely to
occur and a redundant coverage data upload shouldn't cause any problems.

Author: per1234

.github/workflows/test-go-task.yml

@@ -85,7 +85,7 @@ jobs:
         run: task go:test
 
       - name: Send unit tests coverage to Codecov
-        if: matrix.operating-system == 'ubuntu-latest'
+        if: runner.os == 'Linux'
         uses: codecov/codecov-action@v2
         with:
           file: ./coverage_unit.txt