[golang] explicitly set toolchain version (#11665)

Since we're operating inside of the main Go tree, we end up using the
tip go.mod file, which typically defines a Go toolchain version which is
not available yet. Explicitly setting GOTOOLCHAIN to "local" tells the
toolchain to use the right (available) version, instead of trying to
download the tip version.

This should fix the build... I think.

Author: rolandshoemaker

projects/golang/build.sh

@@ -22,6 +22,8 @@ then
 
 fi
 
+export GOTOOLCHAIN="local"
+
 export FUZZ_ROOT="github.com/dvyukov/go-fuzz-corpus"
 
 cd $SRC/text