dashboard: do not add -standard-8 or -standard-16 to AWS machine types

Reported buildlet failure says err=unknown EC2 vm type: m6g.xlarge-standard-16.
Oops.

Change-Id: I510bbb4a5bbf83f39d95305011c2d294cf876be5
Reviewed-on: https://go-review.googlesource.com/c/build/+/420755
Run-TryBot: Russ Cox <[email protected]>
Auto-Submit: Russ Cox <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: rsc

dashboard/builders.go

@@ -1331,8 +1331,11 @@ func (c *BuildConfig) GorootFinal() string {
 	return "/usr/local/go"
 }
 
-// MachineType returns the GCE machine type to use for this builder.
+// MachineType returns the AWS or GCE machine type to use for this builder.
 func (c *HostConfig) MachineType() string {
+	if c.IsEC2() {
+		return c.machineType
+	}
 	typ := c.machineType
 	if typ == "" {
 		if c.NestedVirt || c.MinCPUPlatform != "" {