README: update piglit instructions
README: update piglit instructions

Piglit now has a top level "piglit" command and the location of the
tests can now be read from an environment variable.

Signed-off-by: Thomas Wood <b9025d6a953b68f894a0f6a80fd2af6d55b0badd@intel.com>

chenxianqin/intel-gpu-tools,dlespiau/checkmate-test-igt,dlespiau/checkmate-test-igt,tiagovignatti/intel-gpu-tools,erikarn/intel-gpu-tools,dlespiau/checkmate-test-igt,yipdw/intel-gpu-tools,chenxianqin/intel-gpu-tools,erikarn/intel-gpu-tools,yipdw/intel-gpu-tools,dlespiau/checkmate-test-igt,mv0/intel-gpu-tools,mv0/intel-gpu-tools,mv0/intel-gpu-tools,erikarn/intel-gpu-tools,chenxianqin/intel-gpu-tools,yipdw/intel-gpu-tools,tiagovignatti/intel-gpu-tools,chenxianqin/intel-gpu-tools,mv0/intel-gpu-tools,tiagovignatti/intel-gpu-tools,erikarn/intel-gpu-tools,yipdw/intel-gpu-tools,tiagovignatti/intel-gpu-tools