Merge bitcoin#30185: guix: show *_FLAGS variables in pre-build output

5f2c1d8 guix: show *_FLAGS variables in pre-build output (fanquake)

Pull request description:

  For example:
  ```bash
  # ADDITIONAL_GUIX_COMMON_FLAGS set in the ENV
  ADDITIONAL_GUIX_ENVIRONMENT_FLAGS="--emulate-fhs" ./contrib/guix/guix-build
  <snip>
  INFO: Building f751991 for platform triple x86_64-linux-gnu:
        ...using reference timestamp: 1716905119
        ...running at most 10 jobs
        ...from worktree directory: '/bitcoin'
            ...bind-mounted in container to: '/bitcoin'
        ...in build directory: '/bitcoin/guix-build-f75199182133/distsrc-f75199182133-x86_64-linux-gnu'
            ...bind-mounted in container to: '/distsrc-base/distsrc-f75199182133-x86_64-linux-gnu'
        ...outputting in: '/bitcoin/guix-build-f75199182133/output/x86_64-linux-gnu'
            ...bind-mounted in container to: '/outdir-base/x86_64-linux-gnu'
        ADDITIONAL FLAGS (if set)
            ADDITIONAL_GUIX_COMMON_FLAGS: --no-substitutes
            ADDITIONAL_GUIX_ENVIRONMENT_FLAGS: --emulate-fhs
            ADDITIONAL_GUIX_TIMEMACHINE_FLAGS:
  ```

ACKs for top commit:
  hebasto:
    ACK 5f2c1d8.

Tree-SHA512: 85a6d508499b4ec1d6166343a1707b682d327b2fcfb2fb438571894478aac0062d21e1239b5092091ff98711c5c747151973c4f325a7a7c447d0e807166fcb07

Author: fanquake

contrib/guix/guix-build

@@ -361,6 +361,10 @@ INFO: Building ${VERSION:?not set} for platform triple ${HOST:?not set}:
           ...bind-mounted in container to: '$(DISTSRC_BASE=/distsrc-base && distsrc_for_host "$HOST")'
       ...outputting in: '$(outdir_for_host "$HOST")'
           ...bind-mounted in container to: '$(OUTDIR_BASE=/outdir-base && outdir_for_host "$HOST")'
+      ADDITIONAL FLAGS (if set)
+          ADDITIONAL_GUIX_COMMON_FLAGS: ${ADDITIONAL_GUIX_COMMON_FLAGS}
+          ADDITIONAL_GUIX_ENVIRONMENT_FLAGS: ${ADDITIONAL_GUIX_ENVIRONMENT_FLAGS}
+          ADDITIONAL_GUIX_TIMEMACHINE_FLAGS: ${ADDITIONAL_GUIX_TIMEMACHINE_FLAGS}
 EOF
 
         # Run the build script 'contrib/guix/libexec/build.sh' in the build