ci(conan): disable boost cobalt to unblock fresh boost/1.90.0 builds (#72)

The boost/1.90.0 conan-center recipe fails at package_info() with
"boost_cobalt_io_ssl built but not used in any boost module" — a
recipe-level module-map regression that breaks every fresh boost build
across all coin worktrees (conanfile pins boost/1.90.0 uniformly).

c2pool uses zero cobalt (grep-clean in src/), so without_cobalt=True is
functionally invisible and keeps boost at 1.90.0. Validated green with a
standalone conan install carrying the option.

Harden CI merge-gate ahead of the V36 window (06-09).

Author: frstrtr

conanfile.txt

@@ -7,6 +7,7 @@ leveldb/1.23
 
 [options]
 boost/*:without_exception=False
+boost/*:without_cobalt=True
 
 [generators]
 CMakeDeps