ci: fix breakage because fmtlib changed 'master' to 'main' (#2103)

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

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

Author: lgritz

.github/workflows/ci.yml

@@ -436,7 +436,7 @@ jobs:
             cc_compiler: gcc-14
             cxx_compiler: g++-14
             cxx_std: 20
-            fmt_ver: master
+            fmt_ver: main
             opencolorio_ver: main
             openexr_ver: main
             openimageio_ver: main