rpm/rootfs.py: ensure exit 1 has a word boundary

rpm/rootfs.py: ensure exit 1 has a word boundary

Currently, If a package installed by dnf has the word "exit" followed by
"100%" in the log file, the rpm/rootfs.py regex matches a failure thanks to
the "exit 1"00%, such as the following:

[682/932] perl-module-test2-plugin-exit 100% |   0.0   B/s |   8.5 KiB |  00m00s

Add a `\b` to ensure there's a word boundary to prevent the error from happening.

Signed-off-by: Adam Duskett <adam.duskett@amarulasolutions.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>