Fix Jenkin build failure

Summary:

Previously I changed the line `source ./fbcode.gcc471.sh` to `source fbcode.gcc471.sh`. It works in my devbox but failed in some jenkin servers. I revert the previous code to make sure it works well under all circumstances.

Test Plan:

Test in the jenkin server as well as dev box.

Reviewers:

CC:

Task ID: #

Blame Rev:

build_tools/build_detect_platform

@@ -31,9 +31,9 @@ fi
 # Default to fbcode gcc on internal fb machines
 if [ -d /mnt/gvfs/third-party -a -z "$CXX" ]; then
     if [ -z "$USE_CLANG" ]; then
-        source fbcode.gcc471.sh
+        ./source fbcode.gcc471.sh
     else
-        source fbcode.clang31.sh
+        ./source fbcode.clang31.sh
     fi
 fi