test amber for all bash versions

[b1ek]

we should select a minimal supported version, and run all tests on all bash versions, after the minimal supported one. i think its reasonable to support every bash version after 4.0, as it supports hashmaps which we are likely to implement. it is also since 2009, which means it has broad support on most machines

i suggest we download all bash versions after 4.0 from the bash https server, and then execute tests bwrapped with each version, like this:

run_for_ver() {
    curl https://ftp.gnu.org/gnu/bash/$1.tar.gz
    tar xzf $1.tar.gz
    bwrap --bind $1/bin/bash:/usr/bin/bash cargo r -r
    rm -r $1
}

run_for_ver 4.0
run_for_ver 4.1
...
run_for_ver 5.3

[Mte90]

Well it is something that is not very handy and fast... Maybe it is enough the last 4.x and the last 5.x.

Also can be useful a CI that can be turned on not always but just when it is time to do a new release.