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/abigen/main.go

@@ -33,14 +33,6 @@ import (
 	"github.com/urfave/cli/v2"
 )
 
-var (
-	// Git SHA1 commit hash of the release (set via linker flags)
-	gitCommit = ""
-	gitDate   = ""
-
-	app *cli.App
-)
-
 var (
 	// Flags needed by abigen
 	abiFlag = &cli.StringFlag{
@@ -82,8 +74,9 @@ var (
 	}
 )
 
+var app = flags.NewApp("Ethereum ABI wrapper code generator")
+
 func init() {
-	app = flags.NewApp(gitCommit, gitDate, "ethereum checkpoint helper tool")
 	app.Name = "abigen"
 	app.Flags = []cli.Flag{
 		abiFlag,