ci: explicitly change oom-{score}-adj before running tests

ci: explicitly change oom-{score}-adj before running tests

For some reason root in GH actions is able to _decrease_ its oom score
even after dropping all capabilities (including CAP_SYS_RESOURCE), until
the oom score is changed explicitly after sudo:

$ systemd-detect-virt
microsoft
$ sudo su -
~# capsh --drop=all -- -c 'capsh --print; grep -H . /proc/self/oom*; choom -p $$ -n -101'
Current: =
Bounding set =
Ambient set =
Current IAB: !cap_chown,!cap_dac_override,!cap_dac_read_search,...,!cap_sys_resource,...,!cap_checkpoint_restore
Securebits: 00/0x0/1'b0
 secure-noroot: no (unlocked)
 secure-no-suid-fixup: no (unlocked)
 secure-keep-caps: no (unlocked)
 secure-no-ambient-raise: no (unlocked)
uid=0(root) euid=0(root)
gid=0(root)
groups=0(root)
Guessed mode: UNCERTAIN (0)
/proc/self/oom_adj:8
/proc/self/oom_score:1000
/proc/self/oom_score_adj:500
pid 22180's OOM score adjust value changed from 500 to -101
~# choom -p $$ -n 500
pid 22027's OOM score adjust value changed from 500 to 500
~# capsh --drop=all -- -c 'capsh --print; grep -H . /proc/self/oom*; choom -p $$ -n -101'
Current: =
Bounding set =
Ambient set =
...
uid=0(root) euid=0(root)
gid=0(root)
groups=0(root)
Guessed mode: UNCERTAIN (0)
/proc/self/oom_adj:8
/proc/self/oom_score:1000
/proc/self/oom_score_adj:500
choom: failed to set score adjust value: Permission denied

I have no idea what's going on, but it breaks
exec-oomscoreadjust-negative.service from test-execute when running
unprivileged.