tests: set GOTOOLCHAIN var for report validation

Set GOTOOLCHAIN directive in order to successfully run tests
from root. Else, go will try and download a family of releases
(of the form 1.x), which are not published binaries.

Signed-off-by: Madhav Jivrajani <madhav.jiv@gmail.com>

tests/robustness/makefile.mk

@@ -1,4 +1,5 @@
 .PHONY: test-robustness-reports
+test-robustness-reports: export GOTOOLCHAIN := go$(shell cat .go-version)
 test-robustness-reports:
 	cd ./tests && go test ./robustness/validate -v --count 1 --run TestDataReports