bug#47786: Several build --keep-failed result in wrong env variables

[Dmitry Matveyev]

Hello,

I use guix on Arch Linux, version
050be36cbf3a42199f64f2e44c59f1cb1b3afab5.

Several invocations of guix build --keep-failed creates directories in
/tmp like this one guix-build-hello-2.10.drv-0 for 1st build and then
guix-build-hello-2.10.drv-1 for 2nd and so on (with last digit
increasing). But environment variables for all of them are set to point
to the very 1st directory.

Reproduce:

$ guix build --check --keep-failed hello
^C
$ guix build --check --keep-failed hello
^C
$ cd /tmp/guix-build-hello-2.10.drv-1/
$ grep PWD environment-variables
export OLDPWD
export PWD="/tmp/guix-build-hello-2.10.drv-0/hello-2.10"

Here although we are in directory /tmp/guix-build-hello-2.10.drv-1/, PWD
is set to .drv-0 directory.

Regards,
Dmitry.