remove spurious colon in PATH

Sourcing `envsetup.sh` two or more times introduces spurious colons
in the PATH environment variable. This has the unintended side effect
of adding the current directory to PATH searches.

This CL removes the spurious colon.

Bug: 403545443
Test: local build, `echo $PATH`
Change-Id: Ifc37d03e52079de28aa14b4e49e0091275f60cc3