Keep previous cmake arguments when using buildstandalonegc (#12108)

Currently, using buildstandalonegc makes ./build.sh discard any
previous values of __cmakearg set explicitly or implicitly. Preserve
those arguments.

Without this fix, the following invocations behave differently:

    $ ./build.sh ignorewarnings buildstandalonegc verbose

    $ ./build.sh buildstandalonegc ignorewarnings verbose

1 files changed, 1 insertions, 1 deletions

diff --git a/build.sh b/build.sh
index f337ae22a5..3412663714 100755
--- a/build.sh
+++ b/build.sh
@@ -812,7 +812,7 @@ while :; do
             fi
             ;;
         buildstandalonegc)
-            __cmakeargs="-DFEATURE_STANDALONE_GC=1 -DFEATURE_STANDALONE_GC_ONLY=1"
+            __cmakeargs="$__cmakeargs -DFEATURE_STANDALONE_GC=1 -DFEATURE_STANDALONE_GC_ONLY=1"
             ;;
         msbuildonunsupportedplatform)
             __msbuildonunsupportedplatform=1