CI: reduce xz compression for non release/nightly jobs

Reduce XZ compression level for regular jobs (it is bumped to 9 for releases
and nightly jobs). In my experiments I've got the following bindist size in
the given time for each compression level (with the quick flavour):

  XZ_OPT        Time    Size
  -9            4m06s   112 MB
  -8            4m00s   114 MB
  -7            3m50s   116 MB
  -6 (default)  3m40s   118 MB
  -5            2m47s   123 MB
  -4            1m57s   134 MB
  -3            1m03s   129 MB
  -2            49.73s  136 MB
  -1            37.72s  142 MB
  -0            34.40s  156 MB

.gitlab-ci.yml

@@ -16,6 +16,24 @@ variables:
 
   GIT_SUBMODULE_STRATEGY: "recursive"
 
+  # Reduce XZ compression level for regular jobs (it is bumped to 9 for releases
+  # and nightly jobs). In my experiments I've got the following bindist size in
+  # the given time for each compression level (with the quick flavour):
+  #
+  #     XZ_OPT        Time    Size
+  #     -9            4m06s   112 MB
+  #     -8            4m00s   114 MB
+  #     -7            3m50s   116 MB
+  #     -6 (default)  3m40s   118 MB
+  #     -5            2m47s   123 MB
+  #     -4            1m57s   134 MB
+  #     -3            1m03s   129 MB
+  #     -2            49.73s  136 MB
+  #     -1            37.72s  142 MB
+  #     -0            34.40s  156 MB
+  #
+  XZ_OPT: "-1"
+
 stages:
   - tool-lint      # Source linting of the tools
   - quick-build    # A very quick smoke-test to weed out broken commits
@@ -53,6 +71,8 @@ workflow:
     - if: '$CI_PIPELINE_SOURCE == "web"'
 
 .nightly: &nightly
+  variables:
+    XZ_OPT: "-9"
   rules:
     - if: $NIGHTLY
   artifacts:
@@ -62,6 +82,7 @@ workflow:
 .release: &release
   variables:
     BUILD_FLAVOUR: "perf"
+    XZ_OPT: "-9"
   artifacts:
     when: always
     expire_in: 1 year