ci: don't use extensions on msan build

The memory sanitizer builds are special snowflakes; let them be c90 with
extensions.

Author: ethomson

.github/workflows/main.yml

@@ -132,7 +132,7 @@ jobs:
           env:
             CC: clang
             CFLAGS: -fsanitize=memory -fsanitize-memory-track-origins=2 -fsanitize-blacklist=/home/libgit2/source/script/sanitizers.supp -fno-optimize-sibling-calls -fno-omit-frame-pointer
-            CMAKE_OPTIONS: -DCMAKE_PREFIX_PATH=/usr/local/msan -DUSE_HTTPS=mbedTLS -DUSE_SHA1=HTTPS -DREGEX_BACKEND=pcre -DDEPRECATE_HARD=ON -DUSE_BUNDLED_ZLIB=ON -DUSE_SSH=ON
+            CMAKE_OPTIONS: -DC_EXTENSIONS=ON -DCMAKE_PREFIX_PATH=/usr/local/msan -DUSE_HTTPS=mbedTLS -DUSE_SHA1=HTTPS -DREGEX_BACKEND=pcre -DDEPRECATE_HARD=ON -DUSE_BUNDLED_ZLIB=ON -DUSE_SSH=ON
             CMAKE_GENERATOR: Ninja
             SKIP_SSH_TESTS: true
             SKIP_NEGOTIATE_TESTS: true