Set alsa environment to avoid loading host configuration.

If either `ALSA_CONFIG_DIR` or `ALSA_CONFIG_PATH` are defined on host
system, this causes problems with Flatpak since alsa will try to load
those paths. We now set their values so that they are not picked
by the host.

We also provide an alternative configuration for Flatpak which does
not load "~/.asoundrc" which could be a host configuration in case of
"--filesystem=home". Instead we load `$XDG_CONFIG_HOME/asound.conf`
which is safe because reising on the `.var/app/<id>/config` directory.

Fixes #857
11 jobs for valentindavid/alsa-lib-flatpak-paths-2 in 247 minutes and 5 seconds (queued for 1 second)
Status Job ID Name Coverage
  Flatpak
passed #293351886
aarch64
app_aarch64

02:14:43

passed #293351887
armhf
app_arm

00:56:56

passed #293398203
x86_64 cache_i686
app_i686

01:56:12

passed #293351888
ppc64le allowed to fail
app_powerpc64le

01:34:18

passed #293351884
cache_x86_64 x86_64
app_x86_64

01:17:36

failed #293351885
cache_i686 x86_64
app_i686

01:09:52

 
  Vm
manual #293351892
aarch64 allowed to fail manual
minimal_systemd_vm_aarch64
passed #293351895
armhf
minimal_systemd_vm_arm

00:02:47

passed #293351890
cache_i686 x86_64
minimal_systemd_vm_i686

00:09:27

manual #293351896
ppc64le allowed to fail manual
minimal_systemd_vm_powerpc64le
passed #293351889
cache_x86_64 x86_64
minimal_systemd_vm_x86_64

00:01:21