CI: don't disable -Werror for Julia builds #5788

Job	Run time
Cross compiler for s390x	28m 0s
Test in OpenBSD	7m 58s
testinstall - - ubuntu-latest	4m 27s
teststandard - - ubuntu-latest	20m 24s
testmanuals - - ubuntu-latest	6m 0s
testexpect testmockpkg testspecial test-compile testlibgap testkernel testworkspace - - ubuntu-latest	6m 58s
testpackages testinstall-loadall - - ubuntu-latest	26m 18s
makemanuals testmakeinstall - CONFIGFLAGS="--prefix=/tmp/gapprefix" NO_COVERAGE=1 - ubuntu-latest	5m 23s
teststandard - ABI=32 CONFIGFLAGS="" - ubuntu-20.04	18m 24s
testinstall - HPCGAP=yes ABI=64 - ubuntu-latest	7m 30s
testmockpkg testinstall - BOOTSTRAP_MINIMAL=yes - macos-latest	3m 41s
testbugfix - CONFIGFLAGS="--enable-memory-checking" - ubuntu-latest	27m 21s
testbuildsys testmockpkg testinstall - NO_COVERAGE=1 ABI=64 BUILDDIR=out-of-tree CONFIGFLAGS="--enable-valgrind" - ubuntu-latest	4m 42s
testbuildsys testmockpkg testinstall - NO_COVERAGE=1 ABI=32 BUILDDIR=out-of-tree CONFIGFLAGS="" - ubuntu-20.04	4m 21s
testinstall - JULIA=yes CONFIGFLAGS="--enable-debug" - ubuntu-20.04	51s
testmockpkg testinstall - - windows-2019	16m 30s
Send Slack notification on status change	4s
	3h 8m 52s

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