enables 'buck2 killall' to work when the binary has been deleted

[KlotzAndrew]

This change makes buck2 killall work on linux when the underlying binary has been deleted.

Currently on linux if the buck2 binary has been deleted, buck2 killall returns "No buck2 processes found" and fails to kill the buck2 process. This is because we check the symlinked path in /proc/<pid>/exec for the string 'buck2'. Linux appends the string (deleted) to the symlinked path in the case the binary was deleted, causing the is_buck2_exe check to fail

[meta-codesync[bot]]

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