fix(taskfile): detect Go version from go directive as fallback

Renovate now updates the `go` directive in `go.mod` directly and drops
the redundant `toolchain` line, which broke the `GO_VERSION` lookup in
the Taskfile and caused `task ci` to fail with an empty version.

Read the `toolchain` line when present and fall back to the `go`
directive otherwise. Update the Dockerfile comments to match.

Signed-off-by: Leonardo Cecchi <leonardo.cecchi@enterprisedb.com>

Taskfile.yml

@@ -7,7 +7,7 @@ output: prefixed
 # Variables that are shared across tasks.
 vars:
   GO_VERSION:
-    sh: sed -n 's/^toolchain go//p' go.mod
+    sh: sed -n 's/^toolchain go//p' go.mod | grep . || sed -n 's/^go //p' go.mod
   # renovate: datasource=docker depName=kindest/node versioning=semver
   E2E_KUBERNETES_VERSION: v1.35.1
   E2E_CLUSTER_NAME: barman-cloud-plugin-e2e-{{.E2E_KUBERNETES_VERSION}}

containers/Dockerfile.plugin

@@ -1,5 +1,5 @@
 # Build the manager binary
-# GO_VERSION must be passed as a build arg (read from go.mod toolchain directive by Taskfile)
+# GO_VERSION must be passed as a build arg (read from go.mod by Taskfile)
 ARG GO_VERSION
 FROM --platform=$BUILDPLATFORM golang:${GO_VERSION} AS gobuilder
 ARG TARGETOS

containers/Dockerfile.sidecar

@@ -5,7 +5,7 @@
 # Both components are built before going into a distroless container
 
 # Build the manager binary
-# GO_VERSION must be passed as a build arg (read from go.mod toolchain directive by Taskfile)
+# GO_VERSION must be passed as a build arg (read from go.mod by Taskfile)
 ARG GO_VERSION
 FROM --platform=$BUILDPLATFORM golang:${GO_VERSION} AS gobuilder
 ARG TARGETOS