Bug 1664537 - Don't clobber CFLAGS in asan-fuzzing-ccov config r=marco

We should preserve e.g. the `-fcrash-diagnostics-dir` that is included indirectly via the sourcing of `mozconfig.asan` at line 14.

Differential Revision: https://phabricator.services.mozilla.com/D89952

browser/config/mozconfigs/linux64/asan-fuzzing-ccov

@@ -21,9 +21,9 @@ ac_add_options --disable-profiling
 ac_add_options --disable-warnings-as-errors
 ac_add_options --enable-coverage
 
-export CFLAGS="--coverage"
+export CFLAGS="$CFLAGS --coverage"
-export CXXFLAGS="--coverage"
+export CXXFLAGS="$CXXFLAGS --coverage"
-export LDFLAGS="--coverage -Wl,--compress-debug-sections=zlib"
+export LDFLAGS="$LDFLAGS --coverage -Wl,--compress-debug-sections=zlib"
 
 # gold is required for libFuzzer to work properly
 ac_add_options --enable-linker=gold