check nightlies/pr features with etag avalibility

[johnnychen94]

Users in China often don't have a stable connection to AWS S3, so mirror sites like https://mirrors.tuna.tsinghua.edu.cn exist; they are typically simple static-file HTTP servers and don't have advanced S3-like features such as etags.

Previously, we have provided the JULIAUP_SERVER feature to change the download baseurl, but at that time, I didn't realize that the server mirror should support ETag.

This commit introduces a server feature check operation for custom JULIAUP_SERVER:

• if JULIAUP_SERVER equals the default official ones, it works as usual. No extra overhead.
• Otherwise, send a HEAD check request and set the NIGHTLY_SERVER_SUPPORTS_ETAG feature flag

If this feature flag is false, then the PR/nightlies download will be disabled. This doesn't affect the mirror sites because they don't support mirroring Julia nightlies deliberately; too few people use it, and it consumes too much unnecessary sync bandwidth.

Known JuliaUp mirror sites in China that can be used to test:

https://mirrors.ustc.edu.cn/juliaup-releases
https://mirrors.tuna.tsinghua.edu.cn/juliaup-releases https://mirrors.bfsu.edu.cn/juliaup-releases

This commit makes it work for the above mirror sites and thus fixes #917.

Disclaimer: This commit is primarily generated by AI (cursor + claude 4.5 opus). I've tested it locally on Windows and Linux, and it works as expected. MacOS is not tested.

Example: PR/nightlies features are disabled with a proper error message:

❯ target/debug/juliaup add nightly
Error: The configured nightly server does not support etag headers, which are required for nightly and PR channels.
Nightly and PR channels cannot be installed from this server.

❯ target/debug/julia +nightly
Question: The Juliaup channel 'nightly' is not installed. Would you like to install it?: Yes (install this time only)
Installing Julia nightly as requested
Error: The configured nightly server does not support etag headers, which are required for nightly and PR channels.
Nightly and PR channels cannot be installed from this server.
Error: The Julia launcher failed to determine the command for the `nightly` channel.

Caused by:
Failed to install channel 'nightly'. juliaup add command failed with exit code: Some(1)

Example: normal release download just works

❯ target/debug/juliaup add 1.10
Checking for new Julia versions
Installing Julia 1.10.10+0.x64.linux.gnu
DEBUG: Starting download from URL: https://mirrors.ustc.edu.cn/julia-releases/bin/linux/x64/1.10/julia-1.10.10-linux-x86_64.tar.gz
Downloading ━━━━━━━━━━━━━━━━━━━━━━━━╸ 165.61 MiB/165.79 MiB eta: 0s
Add Installed Julia channel '1.10'

closes #917