fix a mutability inconsistency of `ExtRepOfObj` values of objects in `IsLetterAssocWordRep` #5690

Job	Run time
Cross compiler for s390x	28m 25s
Test in OpenBSD	8m 27s
testinstall - - ubuntu-latest	4m 20s
teststandard - - ubuntu-latest	19m 40s
testmanuals - - ubuntu-latest	5m 40s
testexpect testmockpkg testspecial test-compile testlibgap testkernel testworkspace - - ubuntu-latest	6m 24s
testpackages testinstall-loadall - - ubuntu-latest	26m 4s
makemanuals testmakeinstall - CONFIGFLAGS="--prefix=/tmp/gapprefix" NO_COVERAGE=1 - ubuntu-latest	7m 4s
teststandard - ABI=32 CONFIGFLAGS="" - ubuntu-20.04	18m 29s
testinstall - HPCGAP=yes ABI=64 - ubuntu-latest	7m 18s
testmockpkg testinstall - BOOTSTRAP_MINIMAL=yes - macos-latest	3m 18s
testbugfix - CONFIGFLAGS="--enable-memory-checking" - ubuntu-latest	28m 7s
testbuildsys testmockpkg testinstall - NO_COVERAGE=1 ABI=64 BUILDDIR=out-of-tree CONFIGFLAGS="--enable-valgrind" - ubuntu-latest	4m 20s
testbuildsys testmockpkg testinstall - NO_COVERAGE=1 ABI=32 BUILDDIR=out-of-tree CONFIGFLAGS="" - ubuntu-20.04	4m 37s
testinstall - JULIA=yes CONFIGFLAGS="--enable-debug --disable-Werror" - ubuntu-20.04	4m 38s
testmockpkg testinstall - - windows-2019	15m 19s
Send Slack notification on status change	0s
	3h 12m 10s

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