Download rsync if using mariner

[ellahathaway]

Fixes dotnet/source-build#4536

Test build (internal Microsoft link)

[am11]

An alternative is to add it to the list together with rest of the dependencies https://github.com/dotnet/dotnet-buildtools-prereqs-docker/blob/4cf84c9cb8d9581f2f1f4e330a8199ede451c424/src/cbl-mariner/2.0/crossdeps/amd64/Dockerfile#L4 (perhaps better?)

(also in azl 3.0 https://github.com/dotnet/dotnet-buildtools-prereqs-docker/blob/4cf84c9cb8d9581f2f1f4e330a8199ede451c424/src/azurelinux/3.0/net9.0/crossdeps/amd64/Dockerfile#L4)

[ellahathaway]

An alternative is to add it to the list together with rest of the dependencies https://github.com/dotnet/dotnet-buildtools-prereqs-docker/blob/4cf84c9cb8d9581f2f1f4e330a8199ede451c424/src/cbl-mariner/2.0/crossdeps/amd64/Dockerfile#L4 (perhaps better?)

(also in azl 3.0 https://github.com/dotnet/dotnet-buildtools-prereqs-docker/blob/4cf84c9cb8d9581f2f1f4e330a8199ede451c424/src/azurelinux/3.0/net9.0/crossdeps/amd64/Dockerfile#L4)

I'm going to do this in the Dockerfiles, especially since these changes failed with Error(1601) : Operation not permitted. You have to be root. Thanks for pointing out which Dockerfiles this pool uses.

[am11]

Please double check if the dockerfile is relevant since I can't see internal Microsoft's pipeline. Hope it's the correct one, but we never know. 😁

[ellahathaway]

Nevermind, it's a VM so I can't edit the dockerfile - reopening this PR