Add go-cache-plugin vendored package

Package the tailscale/go-cache-plugin GOCACHEPROG binary for el9 and
Fedora 42/43/44. Upstream has no tags, so pin commit 3031b5d and version
by its commit date.

rpms/go-cache-plugin/metadata.yaml

@@ -0,0 +1,22 @@
+name: go-cache-plugin
+github: tailscale/go-cache-plugin
+description: A GOCACHEPROG implementation that backs the Go build cache with S3.
+arch: amd64
+platform: linux
+maintainer: Tailscale
+homepage: https://github.com/tailscale/go-cache-plugin
+license: BSD-3-Clause
+dist_tag: true
+builds:
+- repository:
+  - almalinux/el9
+  image: git.unkin.net/unkin/almalinux9-rpmbuilder:latest
+  release: 1
+  version: 2026.7.22
+- repository:
+  - fedora/42
+  - fedora/43
+  - fedora/44
+  image: git.unkin.net/unkin/almalinux9-rpmbuilder:latest
+  release: 1
+  version: 2026.7.22

rpms/go-cache-plugin/resources/build.sh

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+#!/usr/bin/bash
+set -e
+
+# Upstream publishes no tags or releases; PACKAGE_VERSION dates this commit.
+COMMIT=3031b5d01c50d2748a32c7c9386ea7049883f38e
+
+# go.mod requires go 1.26.1
+export GOTOOLCHAIN=go1.26.1
+
+# Compile the go-cache-plugin binary using Go
+GOBIN=/app go install github.com/tailscale/go-cache-plugin/cmd/go-cache-plugin@${COMMIT}
+
+# Process nfpm.yaml with envsubst
+envsubst < /app/resources/nfpm.yaml > /app/nfpm.yaml
+
+# Build the RPM
+nfpm pkg --config /app/nfpm.yaml --target /app/dist --packager rpm

rpms/go-cache-plugin/resources/nfpm.yaml

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+# nfpm.yaml
+
+name: ${PACKAGE_NAME}
+version: ${PACKAGE_VERSION}
+release: ${PACKAGE_RELEASE}
+arch: ${PACKAGE_ARCH}
+platform: ${PACKAGE_PLATFORM}
+section: default
+priority: extra
+description: "${PACKAGE_DESCRIPTION}"
+
+maintainer: ${PACKAGE_MAINTAINER}
+homepage: ${PACKAGE_HOMEPAGE}
+license: ${PACKAGE_LICENSE}
+
+disable_globbing: false
+
+replaces:
+  - go-cache-plugin
+
+provides:
+  - go-cache-plugin
+
+# Files to include in the package
+contents:
+  - src: /app/go-cache-plugin
+    dst: /usr/bin/go-cache-plugin
+    file_info:
+      mode: 0755
+      owner: root
+      group: root