Pin manifest plugin version (#9381)

**What this PR does / why we need it**:

CI was failing https://drone.grafana.net/grafana/loki/23216/19/2, we
need to apply the changes from #9342
to 2.8.x.

**Which issue(s) this PR fixes**:
Fixes #<issue number>

**Special notes for your reviewer**:

**Checklist**
- [ ] Reviewed the
[`CONTRIBUTING.md`](https://github.com/grafana/loki/blob/main/CONTRIBUTING.md)
guide (**required**)
- [ ] Documentation added
- [ ] Tests updated
- [ ] `CHANGELOG.md` updated
- [ ] Changes that require user attention or interaction to upgrade are
documented in `docs/sources/upgrading/_index.md`

.drone/drone.jsonnet

@@ -389,7 +389,7 @@ local manifest(apps) = pipeline('manifest') {
   steps: std.foldl(
     function(acc, app) acc + [{
       name: 'manifest-' + app,
-      image: 'plugins/manifest',
+      image: 'plugins/manifest:1.4.0',
       settings: {
         // the target parameter is abused for the app's name,
         // as it is unused in spec mode. See docker-manifest.tmpl
@@ -422,7 +422,7 @@ local manifest_ecr(apps, archs) = pipeline('manifest-ecr') {
   steps: std.foldl(
     function(acc, app) acc + [{
       name: 'manifest-' + app,
-      image: 'plugins/manifest',
+      image: 'plugins/manifest:1.4.0',
       volumes: [{
         name: 'dockerconf',
         path: '/.docker',

.drone/drone.yml

@@ -1164,7 +1164,7 @@ name: manifest
 steps:
 - depends_on:
   - clone
-  image: plugins/manifest
+  image: plugins/manifest:1.4.0
   name: manifest-promtail
   settings:
     ignore_missing: false
@@ -1177,7 +1177,7 @@ steps:
 - depends_on:
   - clone
   - manifest-promtail
-  image: plugins/manifest
+  image: plugins/manifest:1.4.0
   name: manifest-loki
   settings:
     ignore_missing: false
@@ -1190,7 +1190,7 @@ steps:
 - depends_on:
   - clone
   - manifest-loki
-  image: plugins/manifest
+  image: plugins/manifest:1.4.0
   name: manifest-loki-canary
   settings:
     ignore_missing: false
@@ -1203,7 +1203,7 @@ steps:
 - depends_on:
   - clone
   - manifest-loki-canary
-  image: plugins/manifest
+  image: plugins/manifest:1.4.0
   name: manifest-loki-operator
   settings:
     ignore_missing: false
@@ -1682,7 +1682,7 @@ steps:
 - depends_on:
   - clone
   - ecr-login
-  image: plugins/manifest
+  image: plugins/manifest:1.4.0
   name: manifest-lambda-promtail
   settings:
     ignore_missing: true
@@ -1764,6 +1764,6 @@ kind: secret
 name: gpg_private_key
 ---
 kind: signature
-hmac: ad2917515a4e42b876443346b4965a89c3983a14867cdb09ef8f41e83f224856
+hmac: e55dd10435cadfde79f6f334a0d9ad45fc78e6568159c6d58673fcdd3e92d44e
 
 ...