fix(ci): scope conan_deps_cxxflags per Conan package pattern

prior shape (bare json list -> 'tools.build:cxxflags=[...]') leaked into
the consumer/rippled build via the conan-generated toolchain
(CMAKE_CXX_FLAGS_INIT). on macOS, where the build action doesn't
override CMAKE_CXX_FLAGS, this would silently apply the workaround flag
to rippled itself.

reshape the input to a json object keyed by Conan package pattern:
  {"grpc/*":["-Wno-..."]}

action emits package-pattern scoped lines:
  grpc/*:tools.build:cxxflags=["-Wno-..."]

flags only apply while building the matching dependency. consumer
toolchain stays clean. python validator rejects the consumer pattern
('&') and malformed shapes.

.github/workflows/xahau-ga-macos.yml

@@ -109,7 +109,7 @@ jobs:
           stdlib: libcxx
           # grpc 1.50.1 trips clang-19+ -Werror=missing-template-arg-list-after-template-kw
           # on Apple Clang. Drop when grpc is bumped past the fix.
-          conan_deps_cxxflags: '["-Wno-missing-template-arg-list-after-template-kw"]'
+          conan_deps_cxxflags: '{"grpc/*":["-Wno-missing-template-arg-list-after-template-kw"]}'
 
       - name: Build
         uses: ./.github/actions/xahau-ga-build

.github/workflows/xahau-ga-nix.yml

@@ -89,7 +89,9 @@ jobs:
                   # grpc 1.50.1 uses `Foo::template Bar(...)` without an
                   # angle-bracket arg list; clang-19+ promoted that to
                   # -Werror. Drop when grpc is bumped past the fix.
-                  "conan_deps_cxxflags": ["-Wno-missing-template-arg-list-after-template-kw"]
+                  "conan_deps_cxxflags": {
+                      "grpc/*": ["-Wno-missing-template-arg-list-after-template-kw"]
+                  }
               },
               {
                   "compiler_id": "clang-14-libstdcxx-gcc11",
@@ -411,7 +413,7 @@ jobs:
           cc: ${{ matrix.cc }}
           cxx: ${{ matrix.cxx }}
           stdlib: ${{ matrix.stdlib }}
-          conan_deps_cxxflags: ${{ matrix.conan_deps_cxxflags && toJson(matrix.conan_deps_cxxflags) || '[]' }}
+          conan_deps_cxxflags: ${{ matrix.conan_deps_cxxflags && toJson(matrix.conan_deps_cxxflags) || '{}' }}
           gha_cache_enabled: 'false' # Disable caching for self hosted runner
 
       - name: Build