cmd/cue: document -ldflags for local release builds

https://cuelang.org/cl/1194044 did away with fallbackVersion,
and we are moving back to a situation where local builds of cmd/cue
contain v0.0.0 pseudo-versions derived from stamped VCS information.
As such, local release builds should go back to using -ldflags=-X,
which many downstreams such as Linux distros were still doing.

I manually verified that building master with the flag works:

    $ go install ./cmd/cue
    $ cue version | sed 1q
    cue version v0.0.0-20240501095423-2b2cc23a5d70
    $ go install -ldflags='-X cuelang.org/go/cmd/cue/cmd.version=v0.12.345' ./cmd/cue
    $ cue version | sed 1q
    cue version v0.12.345

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I91678ad763c584931147bc540f9446ef78622f3a
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1194118
Reviewed-by: Paul Jolly <paul@myitcv.io>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>

README.md

@@ -109,12 +109,19 @@ Using [Homebrew](https://brew.sh), you can install using the CUE Homebrew tap:
 
 #### Install from Source
 
-You need Go 1.21 or later to build CUE from source; follow the instructions at https://go.dev/doc/install.
+You need [Go 1.21 or later](https://go.dev/doc/install) to build CUE from source.
 
 To download and install the `cue` command line tool, run:
 
 	go install cuelang.org/go/cmd/cue@latest
 
+You can also build the tool locally from source via `go install ./cmd/cue`.
+Note that local release builds [lack version information](https://go.dev/issue/50603),
+so they should inject the version string, such as:
+
+	git switch -d v0.9.0
+	go install -ldflags='-X cuelang.org/go/cmd/cue/cmd.version=v0.9.0' ./cmd/cue
+
 ### Learning CUE
 
 The fastest way to learn the basics is to follow the

cmd/cue/cmd/version.go

@@ -38,13 +38,9 @@ func newVersionCmd(c *Command) *cobra.Command {
 	return cmd
 }
 
-// version can be set by a builder using
-// -ldflags='-X cuelang.org/go/cmd/cue/cmd.version=<version>'.
-// However, people should prefer building via a mechanism which
-// resolves cuelang.org/go as a dependency (and not the main
-// module), in which case the version information is determined
-// from the *debug.BuildInfo (see below). So this mechanism is
-// considered legacy.
+// version can be set at build time to inject cmd/cue's version string,
+// particularly necessary when building a release locally.
+// See the top-level README.md for more details.
 var version string
 
 func runVersion(cmd *Command, args []string) error {