with mathtext.fontset = "cm", \mathbf doesn't make *digits* bold #15338

Bug report

Bug summary

When using computer modern mathtext, \mathbf (actuall, also \mathsf) has no effects on digits.

Code for reproduction

rcParams["mathtext.fontset"] = "cm"; figtext(.5, .5, r"$\mathbf{foo123}$")

Actual outcome

Expected outcome

bold digits, as if passing usetex = True:

Matplotlib version

• Operating system: linux
• Matplotlib version: master, as far back as 2.0 at least
• Matplotlib backend (print(matplotlib.get_backend())): agg
• Python version: 37
• Jupyter version (if applicable):
• Other libraries: