[rstudio-server] allow user to override Ubuntu codename

[HenningLorenzen-ext-bayer]

Follow-up to #232:

1. Posit publishes builds for Ubuntu "Noble" 🎉 .
2. The .deb files for Noble and Jammy are currently the same 🤔 as revealed in rstudio dailies for jammy and noble both point to jammy files rstudio/rstudio#15151.
3. "Resolute" (26.04) development has just started, so everybody should be fine for a while anyway.

devcontainer-features/src/rstudio-server/install.sh

Lines 114 to 116 in 7c3530e

	if [ "${ubuntu_codename}" = "noble" ]; then
	ubuntu_codename="jammy"
	fi

I think it would be nice to derive the download URL straight from the code name in use by the system and allow the user to resolve the situation by explicitly specifiying a code name for that purpose.

[eitsupi]

Thanks, that makes sense.
Contributions are welcome!