[misc] Pin the toolchain gomobile init needs for gobind (#7291)

* [misc] Let gomobile init fetch the toolchain gobind needs

The previous commit's CI run confirmed the failure on a comment-only diff
off main, so the cause is not any branch's changes:

    Android / Build   failure
    iOS / Build       failure

`gomobile init` re-installs gobind from x/mobile@latest whatever gomobile is
pinned to, and setup-go sets GOTOOLCHAIN=local, so the install dies the
moment @latest declares a newer Go than go.mod does:

    gomobile: go install golang.org/x/mobile/cmd/gobind@latest failed: exit status 1
    go: golang.org/x/mobile@v0.0.0-20260821190718-4776eadac327
       requires go >= 1.26.0 (running go 1.25.12; GOTOOLCHAIN=local)

GOTOOLCHAIN=auto on that step alone lets the install fetch what it asks for.
Scoped to the step deliberately: the repo's Go version and every build below
it stay on go.mod's toolchain, so this buys the mobile jobs nothing except
the ability to run gobind.

Pinning gobind next to gomobile does not work — init re-installs @latest
regardless. A durable fix is to stop `init` reaching the network at all, or
to track x/mobile's Go requirement in go.mod; both are larger changes than a
red CI warrants right now.

.github/workflows/mobile-build-validation.yml

@@ -43,8 +43,19 @@ jobs:
         run: /usr/local/lib/android/sdk/cmdline-tools/7.0/bin/sdkmanager --install "ndk;23.1.7779620"
       - name: install gomobile
         run: go install golang.org/x/mobile/cmd/gomobile@v0.0.0-20251113184115-a159579294ab
+      # `gomobile init` re-installs gobind from golang.org/x/mobile@latest
+      # regardless of the pin above (cmd/gomobile/init.go: "Make sure gobind is
+      # up to date"), so this step resolves a version nobody chose, on every run.
+      #
+      # setup-go sets GOTOOLCHAIN=local, so that install fails outright once
+      # x/mobile@latest declares a newer Go than go.mod does — which it did on
+      # 2026-08-21, breaking both jobs on every branch at once. GOTOOLCHAIN=auto
+      # lets this one install fetch the toolchain it asks for. Scoped to the
+      # step: the repo's own Go version, and every build below, is unaffected.
       - name: gomobile init
         run: gomobile init
+        env:
+          GOTOOLCHAIN: auto
       - name: build android netbird lib
         run: PATH=$PATH:$(go env GOPATH) gomobile bind -o $GITHUB_WORKSPACE/netbird.aar -javapkg=io.netbird.gomobile -ldflags="-checklinkname=0 -X golang.zx2c4.com/wireguard/ipc.socketDirectory=/data/data/io.netbird.client/cache/wireguard -X github.com/netbirdio/netbird/version.version=buildtest"  $GITHUB_WORKSPACE/client/android
         env:
@@ -64,8 +75,13 @@ jobs:
           go-version-file: "go.mod"
       - name: install gomobile
         run: go install golang.org/x/mobile/cmd/gomobile@v0.0.0-20251113184115-a159579294ab
+      # See the Android job: `gomobile init` re-installs gobind from
+      # golang.org/x/mobile@latest regardless of the pin above, and needs a
+      # toolchain it may pick newer than go.mod's.
       - name: gomobile init
         run: gomobile init
+        env:
+          GOTOOLCHAIN: auto
       - name: build iOS netbird lib
         run: PATH=$PATH:$(go env GOPATH) gomobile bind -target=ios -bundleid=io.netbird.framework -ldflags="-X github.com/netbirdio/netbird/version.version=buildtest" -o ./NetBirdSDK.xcframework ./client/ios/NetBirdSDK
         env: