Eglot: be careful about gfm-view-mode read-only buffers

Eglot: be careful about gfm-view-mode read-only buffers

Although in most situations this doesn't error, it's only because of
the very wide binding of inhibit-read-only in jsonrpc--process-filter.
That binding will soon be narrowed, so better not rely on it.

Originally reported in
https://github.com/joaotavora/eglot/discussions/1202.

* lisp/progmodes/eglot.el (eglot--format-markup): Inhibit
read-only before touching buffer potentially in gfm-view-mode.