gitlab-ci: test nopcap build with gcc-7 instead of gcc-8

gcc-7 is included with build-essential on Ubuntu 18.04, not installing
an additional gcc-8 saves 110M on disk. The coverage should more or less
be the same, even very old compilers will catch typical build failures.

Change-Id: I03c95dcd1c83b02390489a47de7e2a8d2d757232
Reviewed-on: https://code.wireshark.org/review/36415
Reviewed-by: Dario Lombardo <lomato@gmail.com>
Reviewed-by: Peter Wu <peter@lekensteyn.nl>

.gitlab-ci.yml

@@ -38,12 +38,12 @@
 
 # Rely on fedora:latest and debian-stable jobs for testing a recent GCC version.
 # Since this broke sporadically in the past, test lack of capture support.
-gcc-8-nopcap:
+gcc-7-nopcap:
   <<: *build-ubuntu
   variables:
     CMAKE_ARGS: -DENABLE_PCAP=NO
-    CC: gcc-8
-    CXX: g++-8
+    CC: gcc-7
+    CXX: g++-7
 
 clang-8:
   <<: *build-ubuntu