soulseekqt: don’t linger in a read-only directory after installing

It appears that the strip phase assumes that CWD is writable now (when using
mktemp) and will otherwise fail with:

    mktemp: failed to create file via template 'striperr.XXXXXX': No such file or directory

We now avoid this error by sticking to the build-directory and referencing files
from the AppImage by path.

References: #248569

pkgs/applications/networking/p2p/soulseekqt/default.nix

@@ -28,24 +28,20 @@ mkDerivation rec {
   buildInputs = [ qtmultimedia stdenv.cc.cc ];
 
   installPhase = ''
-      # directory in /nix/store so readonly
-      cd $appextracted
-
-      binary="$(readlink AppRun)"
+      binary="$(realpath ${appextracted}/AppRun)"
       install -Dm755 $binary -t $out/bin
 
       # fixup and install desktop file
       desktop-file-install --dir $out/share/applications \
         --set-key Exec --set-value $binary \
         --set-key Comment --set-value "${meta.description}" \
-        --set-key Categories --set-value Network default.desktop
+        --set-key Categories --set-value Network ${appextracted}/default.desktop
       mv $out/share/applications/default.desktop $out/share/applications/SoulseekQt.desktop
-
       #TODO: write generic code to read icon path from $binary.desktop
-      icon="$(readlink .DirIcon)"
+      icon="$(realpath ${appextracted}/.DirIcon)"
       for size in 16 32 48 64 72 96 128 192 256 512 1024; do
         mkdir -p $out/share/icons/hicolor/"$size"x"$size"/apps
-        convert -resize "$size"x"$size" $icon $out/share/icons/hicolor/"$size"x"$size"/apps/$icon
+        convert -resize "$size"x"$size" $icon $out/share/icons/hicolor/"$size"x"$size"/apps/$(basename $icon)
       done
     '';