Move hooks PATH environment variable to constants

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>