Remove manual pip install commands from tox.ini (#16042)

Having manual calls to `pip` implies that `pip` is available.  This is
not universally true; if the current `venv` and/or `tox` installation is
managed by a different tool that `tox` knows about (such as `uv`), it
will use it to create its own `venv`s, which may not have `pip`
available within them (like `uv`'s don't).  Using the
`dependency-groups` feature allows us to offload all the install work to
`tox`, so it works regardless of `venv` manager.

Technically this causes the dependency groups to install before the
version of Qiskit under test, which will have the side effect of
installing an _old_ version of Qiskit first.  However, `tox` will then
forcibly install the new version of Qiskit into the environment, which
is what we really want.

Author: jakelishman

tox.ini

@@ -87,8 +87,9 @@ basepython = python3
 setenv =
   {[testenv]setenv}
   PYTHON=coverage run --source qiskit --parallel-mode
-commands_pre =
-  pip install -c {toxinidir}/constraints.txt --group optionals-all
+dependency_groups =
+  {[testenv]dependency_groups}
+  optionals-all
 commands =
   stestr run {posargs}
   coverage combine
@@ -102,10 +103,9 @@ allowlist_externals =
 setenv =
   {[testenv]setenv}
   QISKIT_BUILD_PROFILE=debug
-commands_pre =
-  pip install -c {toxinidir}/constraints.txt --group optionals-all
 dependency_groups =
   doc
+  optionals-all
 commands =
   make cheader
   doxygen docs/Doxyfile