[R][CI] Clean up crossbow R templates

[asfimport]

We use a flags param to pass runtime configuration to docker containers

• "flags" is misnamed (since it is typically providing env vars for the docker run phase, e.g. https://github.com/apache/arrow/blob/2d1bd96c951a9c6989c4c475781f59db7987d359/dev/tasks/tasks.yml#L1171)
• Other additional env vars could probably be consolidated into that more transparently (rather than an alternation of CAPITALIZED and lower_case spellings)
• Is there a jinja-way to iterate over a list of params and do things like prepend -e? So that we don't have to form the string "-e FOO=true -e BAR=11"
• Consider using archery docker run instead of docker-compose build ... docker-compose run (cf https://github.com/apache/arrow/blob/master/dev/tasks/docker-tests/azure.linux.yml)

Reporter: Neal Richardson / @nealrichardson

_{Note: This issue was originally created as ARROW-13586. Please see the migration documentation for further details.}

[asfimport]

Krisztian Szucs / @kszucs: @nealrichardson I can't see a PR attached so postponed to 7.0 and removed the in-progress flag.