fix(flux-plunger): correct common-envs.mk include path

The include pointed at ../../../scripts/common-envs.mk, but that
directory does not exist — only hack/common-envs.mk does. flux-plunger
is not part of the root `make build`, so the broken path went unnoticed
until the new image-tags macro made the recipe depend on it.

Without the include, `make -C packages/system/flux-plunger image` fails
immediately with "No such file or directory".

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Signed-off-by: Myasnikov Daniil <myasnikovdaniil2001@gmail.com>

packages/system/flux-plunger/Makefile

@@ -1,7 +1,7 @@
 export NAME=flux-plunger
 export NAMESPACE=cozy-fluxcd
 
-include ../../../scripts/common-envs.mk
+include ../../../hack/common-envs.mk
 include ../../../hack/package.mk
 
 image: