.pytool/EccCheck: Locate BaseTools dir with EDK_TOOLS_PATH

.pytool/EccCheck: Locate BaseTools dir with EDK_TOOLS_PATH

The BaseTools directory is currently being located as a
sub-directory of the WORKSPACE env var. This might not be
true in other environments. Cf EDKII Build Specification,
s4.1.3 "Build Process Restrictions":
  There is no restriction on the location of the EDK_TOOLS_PATH,
  it may be located within a directory identified as the
  WORKSPACE directory, or in any other location that is
  accessible on the development workstation.

Locate the BaseTools directory using EDK_TOOLS_PATH instead.

Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Sami Mujawar <sami.mujawar@arm.com>
Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>