8 AWG solid EGC in raceway for 100 amp subpanel
This isn't an issue because you're dealing with an EGC
As it turns out, while using a stranded wire would have pulled more easily, you are in the clear with regards to your solid 8AWG copper equipment grounding conductor Code-wise.  This is because NEC 310.106(C) (i.e. where your NEC 310.3 cite moved to in newer NEC editions) yields to other parts of the Code when they say things about what kinds of conductors are allowed in raceways:

(C) Stranded Conductors. Where installed in raceways,
conductors 8 AWG and larger, not specifically permitted or
required elsewhere in this Code to be solid, shall be stranded.

and NEC 250.118 point 1 explicitly permits the use of solid or stranded wire for an equipment grounding conductor:

250.118 Types of Equipment Grounding Conductors. The
equipment grounding conductor run with or enclosing the
circuit conductors shall be one or more or a combination of
the following:
(1) A copper, aluminum, or copper-clad aluminum conductor. This conductor shall be solid or stranded; insulated, covered, or bare; and in the form of a wire or a busbar of any shape.

So, relax and enjoy a drink in the meantime, as you have absolutely nothing to worry about.