Move the contents of the PATH environment variable for hooks to
constants, and use its value in the code and in the hooks documentation.

Signed-off-by: Andrea Spadaccini <spadaccio@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>
(cherry picked from commit fe5ca2bb)
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>