fix regex to detect manylinux as ppm binary repo

[AdaemmerP]

This pull request fixes the issues #883 in pak and #183. The problem is that pkgcache:::is_ppm_linux_repo_url() does not recognize manylinux as a binary download url.

[gaborcsardi]

Superficial testing:

> options(repos = c(CRAN = "https://packagemanager.posit.co/cran/__linux__/manylinux_2_28/latest"))
> cmc <- cranlike_metadata_cache$new(bioc = FALSE)
> table(cmc$list()$platform)

aarch64-unknown-linux-gnu-ubuntu-24.04                                 source 
                                 22018                                   1921 

Ideally the platform would be manylinux, but we can improve that later.

[gaborcsardi]

Thank you!