Ensure cppcheck creates a HTML report upon failure

Commit ec72d110 broke the cppcheck job
in GitLab CI: when cppcheck fails, the script is immediately
interrupted, preventing cppcheck-htmlreport from being run.  To ensure
the HTML report is generated when cppcheck fails, revert to invoking
cppcheck-htmlreport in the "after_script" part of the job.

.gitlab-ci.yml

@@ -358,6 +358,7 @@ stages:
     - export GCC_VERSION=$(gcc --version | sed -n 's/.*\([0-9]\+\)\.[0-9]\+\.[0-9]\+.*/\1/p')
     - sed -i "/gcc\",/a\"-DCPPCHECK\", \"-D__STDC__\", \"-D__GNUC__=${GCC_VERSION}\"," compile_commands.json
     - *run_cppcheck
+  after_script:
     - *cppcheck_report_html
   artifacts:
     paths: