utils: Add NIX_STORE_DIR as a candidate for the value of the store directory.

On the daemon side, nixStore gets set to the environment variable
NIX_STORE_DIR, else the environment variable NIX_STORE else the compile time
macro NIX_STORE_DIR (see the Settings::processEnvironment method in
nix/libstore/globals.cc).  When creating a build environment, it sets
NIX_STORE with the value computed as described above.  Hence, it's safer to
look for both NIX_STORE_DIR and NIX_STORE in (guix build utils), so that it
works in any context (build context or external context).

* guix/build/utils.scm (%store-directory): Consider both NIX_STORE_DIR and
NIX_STORE as environment variables.

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