ci: fix breakage because fmtlib changed 'master' to 'main'

fmtlib renamed its "master" branch to "main" yesterday. So now our
"bleeding edge" test that requests fmt "master" now fails.

Signed-off-by: Larry Gritz <lg@larrygritz.com>

Author: lgritz

.github/workflows/ci.yml

@@ -412,7 +412,7 @@ jobs:
             cc_compiler: gcc-14
             cxx_compiler: g++-14
             cxx_std: 23
-            fmt_ver: master
+            fmt_ver: main
             opencolorio_ver: main
             openexr_ver: main
             pybind11_ver: master