build: bump golang: 1.18 -> 1.18.6

Golang Announcement: https://groups.google.com/g/golang-announce/c/x49AQzIVX-s

Notably:
* CVE-2022-27664 and Go issue https://go.dev/issue/54658

Also adding 'opa version' output to the binary smoke tests for easier
verification here.

NOTE That this is not the same commit we've done on main: there, we went from
1.19 to 1.19.1 But since the step from 1.18 to 1.19 is more involved than we'd
like for a patch release, we make an extra step here.

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>

.go-version

@@ -1 +1 @@
-1.18
+1.18.6

Makefile

@@ -416,6 +416,7 @@ endif
 .PHONY: ci-binary-smoke-test-%
 ci-binary-smoke-test-%:
 	chmod +x "$(RELEASE_DIR)/$(BINARY)"
+	"$(RELEASE_DIR)/$(BINARY)" version
 	"$(RELEASE_DIR)/$(BINARY)" eval -t "$*" 'time.now_ns()'
 
 .PHONY: push-binary-edge