Drop the TARBALL_EXTENSION variable

All currently supported BIND 9 branches use xz-packed tarballs for
source code distribution.  Having a variable with a lengthy name that
only holds two characters does not improve readability - it was only
useful for maintaining .gitlab-ci.yml consistency between BIND 9.11 and
all the newer branches, but that era has come to an end a while ago.

Replace all occurrences of the TARBALL_EXTENSION variable in
.gitlab-ci.yml with a fixed string ("xz") to simplify the contents of
that file.

(cherry picked from commit 832c7d4396)

.gitlab-ci.yml

@@ -34,8 +34,6 @@ variables:
 
   UBSAN_OPTIONS: "halt_on_error=1:abort_on_error=1:disable_coredump=0"
 
-  TARBALL_EXTENSION: xz
-
   AM_COLOR_TESTS: always
 
   WITHOUT_READLINE: "--without-readline"
@@ -300,8 +298,8 @@ stages:
 
 # Unpack release tarball and continue work in the extracted directory.
 .unpack_release_tarball: &unpack_release_tarball
-  - tar --extract --file bind-*.tar.${TARBALL_EXTENSION}
-  - rm -f bind-*.tar.${TARBALL_EXTENSION}
+  - tar --extract --file bind-*.tar.xz
+  - rm -f bind-*.tar.xz
   - cd bind-*
 
 .build: &build_job
@@ -691,7 +689,7 @@ tarball-create:
   artifacts:
     paths:
       - diff.patch
-      - bind-*.tar.${TARBALL_EXTENSION}
+      - bind-*.tar.xz
     when: always
   needs:
     - job: autoreconf
@@ -1403,12 +1401,12 @@ release:
   <<: *base_image
   stage: release
   script:
-    - export BIND_DIRECTORY="$(basename bind-*.tar.* ".tar.${TARBALL_EXTENSION}")"
+    - export BIND_DIRECTORY="$(basename bind-*.tar.xz ".tar.xz")"
     # Prepare release tarball contents (tarballs + documentation)
     - mkdir -p "${BIND_DIRECTORY}-release/doc/arm"
     - pushd "${BIND_DIRECTORY}-release"
-    - mv "../${BIND_DIRECTORY}.tar.${TARBALL_EXTENSION}" .
-    - tar --extract --file="${BIND_DIRECTORY}.tar.${TARBALL_EXTENSION}"
+    - mv "../${BIND_DIRECTORY}.tar.xz" .
+    - tar --extract --file="${BIND_DIRECTORY}.tar.xz"
     - mv "${BIND_DIRECTORY}"/{CHANGES*,COPYRIGHT,LICENSE,README.md,srcid} .
     - rm -rf "${BIND_DIRECTORY}"
     - mv "../doc/arm/_build/html" doc/arm/