apt-key fails when POSIXLY_CORRECT environmental variable is set

[Adam Bell]

apt-key fails when the POSIXLY_CORRECT environmental variable is set.
This is due to chmod acting differently with POSIXLY_CORRECT set: chmod tries to use `--` as a parameter with this variable.

This prevents some third-party utilities, like pmrun, from using apt-key.

---

chmod with POSIXLY_CORRECT unset succeeds with the following commands:
```
touch testfile
chmod 0644 -- testfile
echo $?
```
The final command prints "0".
(This is what the apt-key script expects to happen.)

---

chmod with POSIXLY_CORRECT set fails with the following commands:
```
export POSIXLY_CORRECT=
touch testfile
chmod 0644 -- testfile
echo $?
```
The final two commands print these two lines, respectively:
"""
chmod: cannot access '--': No such file or directory
1
"""
(This is unexpected for the apt-key script.)

---

As such, when a third party utility that sets the POSIXLY_CORRECT environmental variable, like pmrun, tries to run apt-key, it will fail.
Example use similar to what the reporting customer tried: `cat some-repo-key.gpg | pmrun apt-key add -`

The chmod examples are run on an up-to-date Bionic container, /usr/bin/apt-key has `chmod #### -- path` commands at line 338 and line 499:
GNU coreutils package version 8.28-1ubuntu1
apt package version 1.6.13

[Julian Andres Klode (juliank)]

Is this a regression from an older release? We can fix it, but if it's not a regression, I'd just skip it, as running apt-key is the wrong approach anyway.

They should just pmrum cp some-repo-key.gpg /etc/apt/trusted.gpg.d. optimally drop the keyring elsewhere and use signed-by in sources.list snippet, but oh well, it does not add much security really.

[Adam Bell (arbell)]

From my several brief tests, this does not appear to be a (recent) regression.

I do see in the apt-key manpage that apt-key is deprecated as of Focal and will be removed after Debian 11 / Ubuntu J/22.04. In the meantime, the `cp some-repo-key.gpg /etc/apt/trusted.gpg.d` method has been brought to the customer as a "workaround" / the new correct method.

The signed-by option is a good idea, too!

Thank you for your response letting us know that a change to apt-key will be skipped.