`BP_VSCODIUM_VERSION` does not allow for selecting the version

[SalimKayal]

While BP_VSCODIUM_VERSION here should allow for selecting vscodium version, it does not.

to reproduce

pack build test-vscodium -B paketobuildpacks/ubuntu-noble-builder-buildpackless -b docker://paketobuildpacks/python -b docker://ghcr.io/swissdatasciencecenter/vscodium-buildpack/vscodium:0.5.0 --env BP_RENKU_FRONTENDS=vscodium --env BP_VSCODIUM_VERSION=1.96.4 --clear-cache --platform linux/amd64 --trust-builder --trust-extra-buildpacks