fix(build): run xgo from project root so its package arg resolves cleanly

xgo treats its last positional arg as a Go package path. Running it from
build/ with `../` confused Go's module resolution — it tried to find a
package inside the build module instead of switching to the project's
own go.mod, failing with:

  package code.vikunja.io/build: build constraints exclude all Go files
  in /go/src/code.vikunja.io/api/build

Use exec.CommandContext with cmd.Dir set to the project's absolute root,
and pass p.BuildPath ("." or "./cmd/veans") as the package arg — the
exact pattern the per-project magefiles used before centralization.
-dest is absolutized so it still ends up under <project>/dist/binaries
regardless of cwd.

Author: kolaente

build/magefile.go

@@ -469,23 +469,38 @@ func xgoOutName(p *project, version string) string {
 	return p.Executable + "-" + versionTagOrUnstable(version)
 }
 
-func runXgo(_ context.Context, p *project, version, targets string) error {
+func runXgo(ctx context.Context, p *project, version, targets string) error {
 	extraLdflags := `-linkmode external -extldflags "-static" `
 	// xgo's darwin builds can't use the static external linker.
 	if strings.HasPrefix(targets, "darwin") {
 		extraLdflags = ""
 	}
-	// xgo's last arg is the path to the Go main package. From build/, the
-	// project's main is at Root/BuildPath.
-	buildTarget := filepath.Join(p.Root, p.BuildPath)
-	return sh.RunV("xgo",
-		"-dest", projectDist(p, subBin),
+	// xgo resolves its last arg as a Go package path. Running it from build/
+	// with `../` confuses the module resolution (it tries to find a package
+	// inside this build module). Invoke xgo from the project root so we can
+	// pass p.BuildPath ("." or "./cmd/veans") just like the original
+	// per-project magefiles did.
+	absRoot, err := filepath.Abs(p.Root)
+	if err != nil {
+		return fmt.Errorf("resolve project root: %w", err)
+	}
+	absDest, err := filepath.Abs(projectDist(p, subBin))
+	if err != nil {
+		return fmt.Errorf("resolve dest dir: %w", err)
+	}
+	//nolint:gosec // mage helper; args are derived from the static project table above.
+	cmd := exec.CommandContext(ctx, "xgo",
+		"-dest", absDest,
 		"-tags", p.BuildTags,
 		"-ldflags", extraLdflags+p.Ldflags(version),
 		"-targets", targets,
 		"-out", xgoOutName(p, version),
-		buildTarget,
+		p.BuildPath,
 	)
+	cmd.Dir = absRoot
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	return cmd.Run()
 }
 
 func xgoAllOS(ctx context.Context, p *project, version string) error {