doc: extensions: doxyrunner: do not modify extension config

After the introduction of #41688, doxyrunner extension modifies the
fmt_vars content (from a variable defined in `conf.py`) when the output
directory variable is defined. This means that Sphinx will always get an
outdated environment and so will always perform a full build instead of
an incremental build. This patch makes a copy first to prevent this
situation.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>

doc/_extensions/zephyr/doxyrunner.py

@@ -185,6 +185,7 @@ def process_doxyfile(
             raise ValueError("Invalid formatting pattern or variables")
 
         if outdir_var:
+            fmt_vars = fmt_vars.copy()
             fmt_vars[outdir_var] = outdir.as_posix()
 
         for var, value in fmt_vars.items():