chore: bump the module path to /v3, take the version from the VCS stamp (#1136)

gitea can not consume the runner's api by version while it's version mismatches the module version:

```
go: gitea.com/gitea/runner@v3.0.1: invalid version: module contains a go.mod file,
so module path must match major version ("gitea.com/gitea/runner/v3")
```

Fix that by bumping the module version now. The existing `v3.0.0` and `v3.0.1` tags stay unusable, so a new tag is needed after this lands.

Also drop the version `-X` linker flags, which would otherwise have to repeat the new path in both `Makefile` and `.goreleaser.yaml`, where a stale path makes injection silently no-op. Go has recorded the module version in the build info since 1.24, so `Version()` reads it from there, keeping the variable as an override for builds without a VCS stamp.

That part started as https://gitea.com/gitea/runner/pulls/1137 but belongs here: the stamp resolves against the tags that are legal for the module path, so without the `/v3` bump it would report `v1.0.9-0.<ts>-<sha>`. Since `release-nightly.yml` triggers on every push to `main`, splitting them would publish a nightly with a `v1` version.

---------

Co-authored-by: bircni <bircni@icloud.com>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1136
Reviewed-by: techknowlogick <9+techknowlogick@noreply.gitea.com>
Reviewed-by: bircni <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>

internal/app/run/proxy.go

@@ -10,7 +10,7 @@ import (
 	"slices"
 	"strings"
 
-	"gitea.com/gitea/runner/act/container"
+	"gitea.com/gitea/runner/v3/act/container"
 
 	log "github.com/sirupsen/logrus"
 	"golang.org/x/net/http/httpproxy"