Remove all stale Go artifacts when upgrading Go version

Also remove /home/gitpod/go (GOPATH with pre-compiled module cache)
in addition to the build cache. The golangci-lint go install was
finding stale .a files compiled with go1.24.9.

Co-authored-by: Ona <no-reply@ona.com>

Author: geropl

dev/image/Dockerfile

@@ -12,12 +12,11 @@ USER root
 # Pin Go version explicitly to ensure all CI-built binaries use a
 # non-vulnerable toolchain (CVE-2025-68121 requires >= 1.24.13).
 ENV GO_VERSION=1.24.13
-RUN rm -rf /usr/local/go \
+RUN rm -rf /usr/local/go /home/gitpod/go /home/gitpod/.cache/go-build \
     && curl -fsSL "https://dl.google.com/go/go${GO_VERSION}.linux-amd64.tar.gz" | tar -C /usr/local -xz \
     && ln -sf /usr/local/go/bin/go /usr/bin/go \
-    && ln -sf /usr/local/go/bin/gofmt /usr/bin/gofmt \
-    && go clean -cache \
-    && rm -rf /home/gitpod/.cache/go-build
+    && ln -sf /usr/local/go/bin/gofmt /usr/bin/gofmt
+ENV GOROOT=/usr/local/go
 ENV PATH=/usr/local/go/bin:$PATH
 
 ### cloud_sql_proxy ###