kwallet-pam: unset QT_PLUGIN_PATH

kwallet sets a limit of 1000 for a single characters for environment
variables read from the socket[1]. wrapQtApps gives us a huge value
for QT_PLUGIN_PATH (up to 13000 bytes on my system!) Since this was
overflowing, the Qt plugin loading mechanism was hitting a segfault
when it was trying to parse the truncated QT_PLUGIN_PATH.

So for now, we can just unset QT_PLUGIN_PATH in the pam_kwallet_init
script. kwalletd5 has its own QT_PLUGIN_PATH which it can use.

This problem occured on 20.03, but not 19.09. It’s unclear what
changes were made in that time, but likely that previously we weren’t
getting a QT_PLUGIN_PATH set in the plasma5 startup at all. This means
that in 19.09 our QT_PLUGIN_PATH value must have been small enough to
fit into the 1000 char limit.

Fixes #77290

[1]: https://github.com/KDE/kwallet/blob/bc9713e2725ab1c4311866f751c674a38584bd92/src/runtime/kwalletd/main.cpp#L44

/cc @ttuegel

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