Slightly refactor new rabbitmqadmin code

- align on "bashbrew architectures" for representing the architecture strings (matching the `Architectures:` lines in our downstream `library/rabbitmq` file; this makes it easier to filter that list later if we so choose)
- avoid multi-stage build, especially to avoid the empty file copying/weirdness
- we always need the `rabbitmqadmin` data, so we should query it before the loop (and pre-process it since the data is the same for every single iteration of the loop)
- store `version` in the `rabbitmqadmin` block (our automation uses this for making nicer commit messages like "Update 4.0 to rabbitmqadmin 2.17.1" instead of just "Update 4.0" which is what the current code will lead to)
- use `dpkg --print-architecture` instead of `uname -m` for architecture detection (the former is "userspace architecture" detection, which more closely matches what we need than the latter's "kernel architecture" detection)

Author: tianon

.gitignore

@@ -1,4 +1 @@
-*.tar.gz
-*.zip
-rustup-init.sh
 .jq-template.awk