Infinite loop in subtyping on 1.6

[KristofferC]

After the backport of #39875 some package started failing with an infinite loop in subtyping:

Internal error: encountered unexpected error in runtime:
StackOverflowError()
jl_gc_alloc at /workspace/srcdir/src/gc.c:3282
jl_alloc_svec_uninit at /workspace/srcdir/src/simplevector.c:60
jl_alloc_svec at /workspace/srcdir/src/simplevector.c:69
save_env at /workspace/srcdir/src/subtype.c:147
forall_exists_subtype at /workspace/srcdir/src/subtype.c:1448
subtype_ccheck at /workspace/srcdir/src/subtype.c:521
var_gt at /workspace/srcdir/src/subtype.c:623
subtype at /workspace/srcdir/src/subtype.c:1259
var_gt at /workspace/srcdir/src/subtype.c:613
subtype at /workspace/srcdir/src/subtype.c:1259
exists_subtype at /workspace/srcdir/src/subtype.c:1426 [inlined]
forall_exists_subtype at /workspace/srcdir/src/subtype.c:1454

I think the problem is with RigidBodyDynamics.jl because the other packages that fail all have that as a dependency: https://s3.amazonaws.com/julialang-reports/nanosoldier/pkgeval/by_hash/d06bab0_vs_a58bdd9/RigidBodyDynamics.1.6.0-rc1-743badd280.log

PkgEval status at https://github.com/JuliaCI/NanosoldierReports/blob/master/pkgeval/by_hash/d06bab0_vs_a58bdd9/report.md.

cc @JeffBezanson