Toolchains for linux/darwin are always downloaded via a third-party mirror

[TheGrizzlyDev]

With the exception of windows, all the other toolchains are currently downloaded from https://github.com/skaji/relocatable-perl . This is potentially an issue as now, even though the rules are authored by this org, this binaries aren't and therefore I have to implicitly trust https://github.com/skaji/relocatable-perl . It would be nice if those archives, like the window one, were mirrored in https://mirror.bazel.build/ .