internal/version: use gitCommit injection in version handling code (#25851)

This changes the CI build to store the git commit and date into package
internal/version instead of package main. Doing this essentially merges our
two ways of tracking the go-ethereum version into a single place, achieving
two objectives:

- Bad block reports, which use version.Info(), will now have the git commit
  information even when geth is built in an environment such as
  launchpad.net where git access is unavailable.

- For geth builds created by `go build ./cmd/geth` (i.e. not using `go run
  build/ci.go install`), git information stored by the go tool is now used
  in the p2p node name as well as in `geth version` and `geth
  version-check`.

cmd/p2psim/main.go

@@ -100,14 +100,8 @@ var (
 	}
 )
 
-var (
-	// Git information set by linker when building with ci.go.
-	gitCommit string
-	gitDate   string
-)
-
 func main() {
-	app := flags.NewApp(gitCommit, gitDate, "devp2p simulation command-line client")
+	app := flags.NewApp("devp2p simulation command-line client")
 	app.Flags = []cli.Flag{
 		apiFlag,
 	}