Makefile: fix DESTDIR handling

[kolyshkin]

DESTDIR should only be used while installing.

To test:

make DESTDIR=$(pwd)/inst PREFIX=/usr install install-man install-bash

Before this commit, this would result in installing to /usr
rather than $(pwd)/usr.

Fixes: #2571

[kolyshkin]

Hmm, I haven't seen this failure before :(

not ok 42 runc exec --additional-gids
# (in test file tests/integration/exec.bats, line 128)
#   `[[ ${output} == "uid=1000 gid=1000 groups=100(users),65534(nogroup)" ]]' failed

and now it's consistently failing everywhere 🤔

[kolyshkin]

OK the expected output is

uid=1000 gid=1000 groups=100(users),65534(nogroup)

while the actual is

uid=1000 gid=1000 groups=100(users),65534(nobody)

Fix is coming

[kolyshkin]

Fix is coming

#2580

[kolyshkin]

rebased on top of current master (merged #2580 should fix CI)

[nanonyme]

Misread. Looks good to me.

[kolyshkin]

@AkihiroSuda PTAL

[kolyshkin]

@AkihiroSuda PTAL