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/ethkey/main.go

@@ -28,14 +28,10 @@ const (
 	defaultKeyfileName = "keyfile.json"
 )
 
-// Git SHA1 commit hash of the release (set via linker flags)
-var gitCommit = ""
-var gitDate = ""
-
 var app *cli.App
 
 func init() {
-	app = flags.NewApp(gitCommit, gitDate, "an Ethereum key manager")
+	app = flags.NewApp("Ethereum key manager")
 	app.Commands = []*cli.Command{
 		commandGenerate,
 		commandInspect,