testscript: fix RequireExplicitExec error messages

They used args[0], which is the first argument to the command,
rather than the command name itself:

	> gofumpt foo.go
	FAIL: [...] use 'exec foo.go' rather than 'foo.go' (because RequireExplicitExec is enabled)

I believe I introduced this regression when refactoring the pull request
due to Roger's review, as I moved the code out of TestMain, where
args[0] was still the name of the command to be run.

Note that I can't easily add a regression test here, because the
testscript command being used in the tests is not a top-level command
run as a separate process, so it does not produce stdout nor stderr.
The change seems easy enough to review manually,
and our tests don't strictly check all error messages anyway.

While here, remove the unnecessary use of -verbose.

testscript/exe.go

@@ -122,7 +122,7 @@ func RunMain(m TestingM, commands map[string]func() int) (exitCode int) {
 			}
 			scriptCmds[name] = func(ts *TestScript, neg bool, args []string) {
 				if ts.params.RequireExplicitExec {
-					ts.Fatalf("use 'exec %s' rather than '%s' (because RequireExplicitExec is enabled)", args[0], args[0])
+					ts.Fatalf("use 'exec %s' rather than '%s' (because RequireExplicitExec is enabled)", name, name)
 				}
 				ts.cmdExec(neg, append([]string{name}, args...))
 			}

testscript/testdata/testscript_explicit_exec.txt

@@ -9,8 +9,8 @@ unquote scripts-explicit/testscript.txt
 testscript scripts-implicit
 testscript scripts-explicit
 
-! testscript -verbose -explicit-exec scripts-implicit
-testscript -verbose -explicit-exec scripts-explicit
+! testscript -explicit-exec scripts-implicit
+testscript -explicit-exec scripts-explicit
 
 -- scripts-implicit/testscript.txt --
 >fprintargs stdout right