buildsys: allow overriding Julia config via env vars (#5830) #5807

Job	Run time
Cross compiler for s390x	27m 34s
testinstall - - ubuntu-latest	4m 34s
teststandard - - ubuntu-latest	19m 5s
testmanuals - - ubuntu-latest	5m 59s
testexpect testmockpkg testspecial test-compile testlibgap testkernel testworkspace - - ubuntu-latest	6m 33s
testpackages testinstall-loadall - - ubuntu-latest	25m 53s
makemanuals testmakeinstall - CONFIGFLAGS="--prefix=/tmp/gapprefix" NO_COVERAGE=1 - ubuntu-latest	5m 40s
teststandard - ABI=32 CONFIGFLAGS="" - ubuntu-20.04	18m 15s
testinstall - HPCGAP=yes ABI=64 - ubuntu-latest	7m 15s
Test in OpenBSD	7m 57s
testmockpkg testinstall - BOOTSTRAP_MINIMAL=yes - macos-latest	4m 1s
testbugfix - CONFIGFLAGS="--enable-memory-checking" - ubuntu-latest	27m 32s
testbuildsys testmockpkg testinstall - NO_COVERAGE=1 ABI=64 BUILDDIR=out-of-tree CONFIGFLAGS="--enable-valgrind" - ubuntu-latest	4m 28s
testbuildsys testmockpkg testinstall - NO_COVERAGE=1 ABI=32 BUILDDIR=out-of-tree CONFIGFLAGS="" - ubuntu-20.04	4m 27s
testinstall - JULIA=yes CONFIGFLAGS="--enable-debug" - ubuntu-20.04	5m 4s
testmockpkg testinstall - - windows-2019	15m 40s
Send Slack notification on status change	3s
	3h 10m 0s

Provide feedback