This phase is inherited by other build systems, which ensures
'SOURCE_DATE_EPOCH' is always set in practice.
* guix/build/python-build-system.scm (set-SOURCE-DATE-EPOCH): Remove.
(%standard-phases): Don't add it.
* guix/build/gnu-build-system.scm (set-SOURCE-DATE-EPOCH): New
procedure.
(%standard-phases): Add it.
(gnu-build): Remove 'setenv' call for "SOURCE_DATE_EPOCH".
* gnu/packages/python.scm (python-2)[arguments]: Set SOURCE_DATE_EPOCH
to 1 to match what the daemon does.
* guix/build/python-build-system.scm (set-SOURCE-DATE-EPOCH): Likewise.
* gnu/packages/patches/python-2.7-source-date-epoch.patch: New file.
* gnu/packages/python.scm (python-2)[source]: Use it.
[arguments]: Set SOURCE_DATE_EPOCH in 'patch-lib-shells' phase.
* guix/build/python-build-system.scm (set-SOURCE-DATE-EPOCH): New
procedure.
(%standard-phases): Add it.
* gnu-system.am (dist_patch_DATA): Add patch.
build system (switches to Python 3) and compute python-version instead of
passing it as a parameter.
* guix/build-system/python.scm (default-python): Switch to python-wrapper.
* guix/build-system/python.scm (python-build): Drop parameter #:python-version.
* guix/build/python-build-system.scm (wrap): Compute python version from input.