go: Drop unnecessary backslash-escape

There's no backslash interpretation going on within single-quote strings
which means there's no need to escape the backslash. Since this was going
on within single-quote strings the $exclude variable ended up having 2
backslashes (`\\`) instead of the intended single backslash. This meant
that the regex that was built up was incorrect. For example prometheus'
exclude contents before and after this change are:

✕: \(/_\|examples\|Godeps\|testdata\\|documentation/prometheus-mixin\)
✓: \(/_\|examples\|Godeps\|testdata\|documentation/prometheus-mixin\)

This is a port of 60543c4f9e

builder/default.nix

@@ -266,7 +266,7 @@ let
             IFS=' ' read -r -a excludedArr <<<$excludedPackages
             printf -v excludedAlternates '%s\\|' "''${excludedArr[@]}"
             excludedAlternates=''${excludedAlternates%\\|} # drop final \| added by printf
-            exclude+='\\|'"$excludedAlternates"
+            exclude+='\|'"$excludedAlternates"
           fi
           exclude+='\)'