pypy3: use valid pypy3 path in src_install

Set $PYTHON to a known existing path for pypy3-c instead of a broken
symlink. The PYTHON env var is used by python_optimize to
determine the path of the python implementation used to compile all the
standard libs, and if the path is broken, .pyc files don't get
generated.

More information about the underlying issue and potential fixes is
available at http://shortn/_kP8nkDxu58.

NOTE: this is a reland of crrev.com/c/3232964 which was rolled back in
crrev.com/c/3241730. The reland is being tried in conjunction with:
1. Bumping the ebuild revision number
2. A chroot hook which unmerges pypy3 and forces a rebuild of the
   package (crrev.com/c/3243334)

BUG=b:201487405,b:187793277
TEST=CQ; chromiumos-sdk-trybot previously succeeded in
https://ci.chromium.org/b/8832717509459089201,
https://ci.chromium.org/b/8832717428707011585.
TEST=new tryjob kicked off with specs `cros tryjob --remote -g 3243334
-g 3243986 -g 3246610 chromiumos-sdk-tryjob' and is running at
https://ci.chromium.org/b/8832260528493254337

Cq-Depend: chromium:3243334, chromium:3243986
Change-Id: If1a6758c437f19bb203d45d946669bb8141c97ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/portage-stable/+/3246610
Auto-Submit: Lizzy Presland <zland@google.com>
Tested-by: Lizzy Presland <zland@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
Commit-Queue: Lizzy Presland <zland@google.com>
1 file changed