utils/wa_results_collector: Change sha1 acquisition method

Created by: valschneider

To map a WA run with a kernel commit sha1, we currently read whatever __meta/target_info.json reports for "kernel_release" (which should be the equivalent of uname -r).

However, there are cases where this doesn't seem to be reported. For instance, a HiKey960 built with hikey960_defconfig will not report the commit sha1 in its version data, which will cause an exception in the results collector code.

Since the sha1 is included in the results directory name anyway, use that instead as a more reliable source.