Improve and update windows text in INSTALL.md #4967

Job	Run time
testinstall - - ubuntu-latest	4m 25s
teststandard - - ubuntu-latest	31m 31s
testmanuals - - ubuntu-latest	5m 57s
testexpect testmockpkg testspecial test-compile testlibgap testkernel testworkspace - - ubuntu-latest	6m 19s
testpackages testinstall-loadall - - ubuntu-latest	21m 15s
makemanuals testmakeinstall - CONFIGFLAGS="--prefix=/tmp/gapprefix" NO_COVERAGE=1 - ubuntu-latest	5m 20s
teststandard - ABI=32 CONFIGFLAGS="" - ubuntu-20.04	28m 40s
testinstall - HPCGAP=yes ABI=64 - ubuntu-latest	7m 11s
testmockpkg testinstall - BOOTSTRAP_MINIMAL=yes - macos-latest	6m 18s
testbugfix - CONFIGFLAGS="--enable-memory-checking" - ubuntu-latest	23m 2s
testbuildsys testmockpkg testinstall - NO_COVERAGE=1 ABI=64 BUILDDIR=out-of-tree CONFIGFLAGS="--enable-valgrind" - ubuntu-latest	3m 52s
testbuildsys testmockpkg testinstall - NO_COVERAGE=1 ABI=32 BUILDDIR=out-of-tree CONFIGFLAGS="" - ubuntu-20.04	2m 54s
testinstall - JULIA=yes CONFIGFLAGS="--enable-debug --disable-Werror" - ubuntu-20.04	4m 29s
testmockpkg testinstall - - windows-2019	15m 48s
Send Slack notification on status change	0s
	2h 47m 1s

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