Allow (or have an option) to have `autoinstallchannels` work with `JULIAUP_CHANNEL`

[samuel-massinon]

Context: We have some CIs that will use juliaup and JULIAUP_CHANNEL to manage the julia version.

The issue is that if the specific julia version is not installed, we get an error like the following

❯ juliaup config autoinstallchannels true
   Configure Property 'autoinstallchannels' set to 'true'

❯ julia +1.9
  Installing Julia 1.9 automatically per juliaup settings
    Checking for new Julia versions
  Installing Julia 1.9.4+0.x64.linux.gnu
 Downloading ╺━━━━━━━━━━━━━━━━━━━━━━━━ 1.42 MiB/139.39 MiB eta: 65s                                                                                         ^C⏎  

❯ set -x JULIAUP_CHANNEL 1.9

❯ julia 
ERROR: `1.9` from environment variable JULIAUP_CHANNEL is not installed. Please run `juliaup add 1.9` to install channel or version.

It would be nice to be able to auto install with JULIAUP_CHANNEL

Current workaround is to manually specify version before running julia

    - name: Make sure julia versions are loaded
      shell: bash
      run: |
        juliaup add 1.12
        juliaup add 1.11