guix: qt-utils: Wrapped executables honor user's envvars.

guix: qt-utils: Wrapped executables honor user's envvars.

Prior to this change, wrappers did set the specified environment variables to
a fixed value, overwriting any user settings. This inhibited propagating
e.g. XDG_DATA_DIRS from a profile to the application.

Now user environment variables are prefixed (if the variable defines some
"binary" search path, e.g. QT_PLUGIN_PATH) or suffixed (if the variable
defines some config or data search path, e.g. XDG_DATA_DIRS). The code could
also allow to overwrite, anyhow currently no variable is defined like this.

* guix/build/qt-utils.scm (variables-for-wrapping): For each env-var to
  be wrapped, specify whether it should prefix, suffix or overwrite the
  user's variable.