Add 'su' to mariner images

[sbomer]

Builds are failing in Azure Pipelines during the "Initialize containers" step with:

##[error]Docker-exec executed: `su -c "echo '%azure_pipelines_sudo ALL=(ALL:ALL) NOPASSWD:ALL' >> /etc/sudoers"`; container id: `2f0b84690195e3b89ee553a19e665382f3ead9d90376bf2062087b08f5a309e0`; exit code: `126`; command output: `OCI runtime exec failed: exec failed: unable to start container process: exec: "su": executable file not found in $PATH: unknown`

(see dotnet/runtime#84148)

Contributes to dotnet/runtime#83428

[sbomer]

@mthalman @janvorli PTAL

[janvorli]

LGTM, thank you!