MeshAgent on Linux should respect the default shell assigned to the user it's running as

[MailYouLater]

When connecting to a Linux computer via the Terminal tab, the mesh agent always launches /bin/bash even if the current user's default shell is something else.

This issue was brought up by @sylvaincolson in Ylianst/MeshCentral#678 because they switched the mesh agent to run under a non-root user that used /bin/rbash as the shell instead of /bin/bash in an attempt to restrict the permissions given to people logging in to the machine through MeshCentral, but the issue would also affect a user who simply wanted to switch from bash to another shell (e.g. fish) by changing the default shell of the user that it logs into by default (which is root).