ci: add support for Go 1.16 (#117)

Involves a quick fix to cmd/testscript tests which currently rely on the
pre 1.16 behaviour of cmd/go to automatically update go.{mod,sum} as
part of any build command.

Adding go1.16beta1 to the build matrix is a function of
github.com/actions/setup-go/issues/96

cmd/testscript/testdata/simple.txt

@@ -11,6 +11,7 @@ stdout 'example.com/mod'
 
 -- file.txt --
 >env GOMODCACHE=$WORK
+>go get -d fruit.com
 >go list -m
 >stdout 'example.com/mod'
 >go list fruit.com/...