CI: fix missing 'OS_ID' env var in ctest jobs

CI: fix missing 'OS_ID' env var in ctest jobs

Jobs inheriting from this template were attempting to set `$OS_ID` from the file
OS_ID that did not exist before `install-package.sh` was run. The result was
output in the build log like:

  export OS_ID=$( cat OS_ID )
  cat: OS_ID: No such file or directory

Somewhat surprisingly, tests were still passing. The reason is that `$OS_ID` is
only accessed in ci/tests.py and on Ubuntu (the platform used for both ctest
jobs) the only `$OS_ID`-based skip is for `mingle`, that is also coincidentally
skipped due to these jobs using the CMake build system where `mingle` is not
build. That is, the mingle check is meant to be skipped on lines 104-108, but
actually ends up skipped on lines 116-122 due to `$OS_ID` being empty.

In future, all of this should be refactored to use a less brittle technique for
asking “what distro are we running on?” When moving to Python 3.10,
`platform.freedesktop_os_release()` would be the obvious candidate.