cmd/buildlet: build with Go 1.20

One of the ports requires 1.20 and I don't think there's any harm in
upgrading. 1.21 might be tricky if it really breaks Windows 2008,
though, since we'll still have to support it until 1.20 ages out.

Change-Id: I0592a0ec9ea5008f9bd19f7bd0990967df43849b
Reviewed-on: https://go-review.googlesource.com/c/build/+/495937
Reviewed-by: Carlos Amedee <[email protected]>
Run-TryBot: Heschi Kreinick <[email protected]>
Auto-Submit: Heschi Kreinick <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: heschi

cmd/buildlet/Makefile

@@ -11,7 +11,7 @@
 #  - upload.linux-amd64: build buildlet for linux-amd64 and upload to prod
 #  - dev-upload.linux-amd64: build buildlet for linux-amd64 and upload to dev cluster
 
-GO=go1.19
+GO=go1.20
 
 ALL=\
 	buildlet.aix-ppc64 \