golang: Add go version 1.18

Go 1.18 is the incremental improvement for earlier go 1.17 release.
Is "is a significant release, including changes to the language,
implementation of the toolchain, runtime, and libraries". Please find
all details in the official release note:

    https://go.dev/doc/go1.18

It was released on 2022-03-15. Due to the go promise of compatibility
most software should still compile without any problems.

/reviewed-on nexedi/slapos!1305

component/golang/buildout.cfg

@@ -81,6 +81,18 @@ md5sum = 4476707f05cf6915ec1173038dc357a9
 environment-extra =
   GOROOT_BOOTSTRAP=${golang14:location}
 
+[golang1.18]
+<= golang-common
+url = https://golang.org/dl/go1.18.9.src.tar.gz
+md5sum = e2caa7c4de49aa77a14c694bfc9a5cd1
+
+# go1.18 needs go1.4 or go1.17 to bootstrap.
+# We use go1.4 to reduce the amount of components which
+# need to be installed. See following note for rationale:
+# https://lab.nexedi.com/nexedi/slapos/merge_requests/1305#note_174936
+environment-extra =
+  GOROOT_BOOTSTRAP=${golang14:location}
+
 # ---- infrastructure to build Go workspaces / projects ----
 
 # gowork is the top-level section that defines Go workspace.