We make sure that Coq lang versions < 0.8 can still somewhat compose with
installed theories. The way this was done before was due to `-boot` not being
passed to Coq. This meant that we essentially have `-Q "" user-contrib` being
passed. In order to restore this behaviour for Coq lang < 0.8 we explicitly add
that flag. This PR makes sure that this is indeed the case.

We configure COQLIB to be lib/coq. Coq will search for user-contrib from here.
We also need to set up a fake Coq install.

  $ mkdir -p lib/coq
  $ export COQLIB=$PWD/lib/coq
  $ echo $COQLIB
  $TESTCASE_ROOT/lib/coq

  $ mkdir -p lib/coq/theories/Init/
  $ cat > lib/coq/theories/Init/Prelude.v << EOF
  > Inductive PreludeLoaded := Yes.
  > EOF

We need to manually compile the prelude.

  $ coqc -boot -noinit -w -deprecated-native-compiler-option -w -native-compiler-disabled -native-compiler no -R lib/coq/theories/ Coq lib/coq/theories/Init/Prelude.v

We also setup some plugins

  $ mkdir -p lib/coq-core/plugins

We setup an installed theory. Note that lib/coq/user-contrib doesn't exist yet,
so this also tests that it won't be a problem.

  $ dune build --root B @install
  Entering directory 'B'
  Warning: Coq Language Versions lower than 0.8 have been deprecated in Dune
  3.8 and will be removed in an upcoming Dune version.
  Hint: To disable this warning, add the following to your dune-project file:
  (warnings (deprecated_coq_lang_lt_08 disabled))
  Leaving directory 'B'
  $ dune install --root B --prefix=$PWD --display=short
  Installing $TESTCASE_ROOT/lib/B/META
  Installing $TESTCASE_ROOT/lib/B/dune-package
  Installing $TESTCASE_ROOT/lib/coq/user-contrib/B/.coq-native/NB_b.cmi
  Installing $TESTCASE_ROOT/lib/coq/user-contrib/B/.coq-native/NB_b.cmxs
  Installing $TESTCASE_ROOT/lib/coq/user-contrib/B/b.glob
  Installing $TESTCASE_ROOT/lib/coq/user-contrib/B/b.v
  Installing $TESTCASE_ROOT/lib/coq/user-contrib/B/b.vo


Next we go into our Dune project and build it.
  $ dune build --root A
  Entering directory 'A'
  Inductive hello : Set :=
      I : hello | am : hello | an : hello | install : hello | loc : hello.
  Leaving directory 'A'

Now we check the flags that were passed to coqdep and coqc:

  $ tail -4 A/_build/log | head -2 | ../scrub_coq_args.sh
  coqdep
  -boot
  -R coq/theories Coq
  -Q $TESTCASE_ROOT/lib/coq/user-contrib/B B
  -R . A -dyndep opt -vos a.v >
  _build/default/.A.theory.d
  coqc -q
  -w -deprecated-native-compiler-option -native-output-dir .
  -native-compiler on
  -nI lib/coq-core/kernel
  -nI .
  -boot
  -R coq/theories Coq
  -Q $TESTCASE_ROOT/lib/coq/user-contrib/B B
  -R . A
  a.v
