Move calculation of gitlab templates version out of *show-ci-variables

Here are two "before" examples of the problem. First running .show-ci-variables in a composer after_script fails, because we are trying to curl the file without the _CURL_TEMPLATES_REPO and REF being set.

Second, adding .show-ci-variables in another job after_script works, but is confusing and wasteful because we are downloading and executing the script again

Downstream pipelines for D10 and D7 show the Gitlab Templates version as required.

Here's a test, where the reference is using in cspell after_script

cspell:
  after_script:
    - !reference [ .show-ci-variables ]
    - echo "=== GITLAB_TEMPLATES_VERSION=$GITLAB_TEMPLATES_VERSION"
    - echo "=== VERSION_USING_PUTENV=$VERSION_USING_PUTENV"
    - echo "=== GITLAB_TEMPLATES_VERSION2=$GITLAB_TEMPLATES_VERSION2"
    - echo "=== VERSION_USING_PUTENV2=$VERSION_USING_PUTENV2"
    - source build.env
    - echo "GITLAB_TEMPLATES_VERSION=$GITLAB_TEMPLATES_VERSION"
    - echo "GITLAB_TEMPLATES_VERSION2=$GITLAB_TEMPLATES_VERSION2"

The value is displayed, but annoyingly it also shows the quotes, as they are written to build.env. But when an extra source build.env is added the variables do not have the quotes. This does not really matter, as the default display in Composer is clean. It is just unusual that the values read from build.env a second time appear to behave better than the initial read.
https://git.drupalcode.org/issue/scheduler-3445052/-/jobs/5085925

I made a couple of comments in the MR, one of them around the quotes, in case it makes a difference.

I've tidied up the try-out code, renamed the script as requested, and removed the quotes. All runs ok in the latest downstream pipeline (the previous failure was before I rebased, and phpunit failed due to the quotes around project type, that was fixed yestterday)

Here's a test where cspell after_script calls !reference [ .show-ci-variables ] and it works cleanly
https://git.drupalcode.org/issue/scheduler-3445052/-/pipelines/486624

This is ready for review.

This looks good to me. Thanks for addressing the feedback. RTBC.

Merged.

Just for the record, the build.env file is read automatically and the variables with spaces and without quotes are loaded correctly in all script sections of the subsequent jobs that follow a composer job. The only place where the variables created in composer: script: are automatically available is in a custom composer: after_script: and this is where source build.env used to work, but now fails due to having a variable with spaces in. But the same can be achieved using the slightly more complicated version IFS=$'\n' && export $(<build.env) && unset IFS

An example of this is https://git.drupalcode.org/issue/scheduler-3445052/-/jobs/5132987#L1122