Actually prepare beta 1.49.0

[emilyalbini]

This PR sets everything up for beta 1.49.0, and backports the following PRs to it:

• build-manifest: strip newline from rustc version #79107 - build-manifest: strip newline from rustc version
• Avoid installing external LLVM dylibs #78986 - Avoid installing external LLVM dylibs
• Install CI llvm into the library directory #79074 - Install CI llvm into the library directory
• Update RELEASES.md for 1.48.0 #78364 - Update RELEASES.md for 1.48.0
• Ensure that the source code display is working with DOS backline #77939 - Ensure that the source code display is working with DOS backline

This is the same as #79132, but actually targeting the right branch.

r? @ghost
cc @rust-lang/release