tools_def.template: Change CLANG_BIN from DEF to ENV

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Olivier Martin <olivier.martin@arm.com>
Reviewed-by: Andrew Fish <afish@apple.com>



git-svn-id: https://svn.code.sf.net/p/edk2-buildtools/code/trunk/BaseTools@2644 c2973edb-eda0-4c78-bc6a-9341b269661f