testing/pmbootstrap: add missing git dependency

pmbootstrap refuses to start without git:

pine64-pinephone:~$ sudo apk add pmbootstrap
(1/15) Installing gdbm (1.13-r1)
(2/15) Installing python3 (3.8.2-r7)
(3/15) Installing libacl (2.2.53-r0)
(4/15) Installing libattr (2.4.48-r0)
(5/15) Installing coreutils (8.32-r0)
(6/15) Installing libproc (3.3.16-r0)
(7/15) Installing procps (3.3.16-r0)
(8/15) Installing openssl (1.1.1g-r0)
(9/15) Installing py3-ordered-set (4.0.1-r0)
(10/15) Installing py3-appdirs (1.4.4-r0)
(11/15) Installing py3-parsing (2.4.7-r0)
(12/15) Installing py3-six (1.14.0-r1)
(13/15) Installing py3-packaging (20.3-r0)
(14/15) Installing py3-setuptools (46.2.0-r0)
(15/15) Installing pmbootstrap (1.18.1-r1)
Executing pmbootstrap-1.18.1-r1.post-install
modprobe: FATAL: Module binfmt_misc not found in directory /lib/modules/5.6.0
Executing busybox-1.31.1-r15.trigger
OK: 1086 MiB in 393 packages

pine64-pinephone:~$ pmbootstrap init
[08:19:23] ERROR: Can't find all programs required to run pmbootstrap. Please install first: git
[08:19:23] See also: <https://postmarketos.org/troubleshooting>

It's also mentionned in the requirements https://gitlab.com/postmarketOS/pmbootstrap/-/tree/master#requirements.

This MR adds the missing dependency.