make: pkg: don't export globally GITCACHE and GIT_CACHE_DIR vars

[aabadie]

Contribution description

Currently git-cache is only used in pkg.mk but the corresponding variables (GITCACHE and GIT_CACHE_DIR) are globally exported by the build system.
This PR cleans up this a little:

• Move definitions of GITCACHE and GIT_CACHE_DIR variables in pkg.mk
• Only export GIT_CACHE_DIR for the target that clones the package
• Remove unnecessary redefinition of GITCACHE in flatc and moquitto_rsmb tools
• Ensure both variables are not exported

Note that it's still possible to set custom GITCACHE and GIT_CACHE_DIR values from the command line.

I might have miss something related to the CI.

Testing procedure

• A green Murdock
• git-cache still functional:
  • enable gitcache: ./dist/tools/git/git-cache init
  • build an application with a package works:

    make -C tests/pkg_lvgl
    

  • build flatc or mosquitto_rsmb works:

    make -C dist/tools/flatc
    

Issues/PRs references

Ticks one item in #10850

[aabadie]

This PR doesn't seem to cause any issue with Murdock. @kaspar030, do you see any issue with the approach ?

[kaspar030]

ACK.

[kaspar030]

Error: Commit message is longer than 72 characters:
	    "Makefile.include: don't define/export GITCACHE and GIT_CACHE_DIR globally"

and please squash!

[aabadie]

Rebased and squashed!