integration/export/triple: use LITEX_ENV_CC_TRIPLE instead of TRIPLE.

triple can be used internally, but is too generic as an environment variable.

litex/soc/integration/export.py

@@ -64,7 +64,7 @@ def get_cpu_mak(cpu, compile_software):
         r = None
         if not isinstance(triple, tuple):
             triple = (triple,)
-        override = os.getenv( "TRIPLE" )
+        override = os.getenv("LITEX_ENV_CC_TRIPLE")
         if override:
             triple = (override,) + triple
         p = get_platform()