Keep https_proxy populated for curl downloads

When building Chromium OS behind a proxy, https_proxy needs to be set
for curl. Additional discussion can be found at [1]. Chromium configures
the chroot FETCHCOMMAND at [2].

Example fetch command:
curl --ipv4 -f -y 30 --retry 9 -L \
        --output /var/cache/chromeos-cache/distfiles/host/zlib-1.2.11.tar.gz \
        https://commondatastorage.googleapis.com/chromeos-localmirror/distfiles/zlib-1.2.11.tar.gz

Example error message:
>>> 13:49:14 === Start output for job zlib-1.2.11 ===
zlib-1.2.11: >>> Downloading 'https://commondatastorage.googleapis.com/chromeos-localmirror/distfiles/zlib-1.2.11.tar.gz'
zlib-1.2.11:   % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
zlib-1.2.11:                                  Dload  Upload   Total   Spent    Left  Speed
    0     0    0     0    0     0      0      0 --:--:--  0:02:10 --:--:--     0
zlib-1.2.11: curl: (7) Failed to connect to commondatastorage.googleapis.com port 443: Connection timed out
zlib-1.2.11: >>> Downloading 'https://commondatastorage.googleapis.com/chromeos-mirror/gentoo/distfiles/zlib-1.2.11.tar.gz'
zlib-1.2.11:   % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
zlib-1.2.11:                                  Dload  Upload   Total   Spent    Left  Speed
    0     0    0     0    0     0      0      0 --:--:--  0:02:11 --:--:--     0
zlib-1.2.11: curl: (7) Failed to connect to commondatastorage.googleapis.com port 443: Connection timed out
zlib-1.2.11: !!! Couldn't download 'zlib-1.2.11.tar.gz'. Aborting.
zlib-1.2.11:  * Fetch failed for sys-libs/zlib-1.2.11, Log file:
zlib-1.2.11:  *   /var/log/portage/sys-libs:zlib-1.2.11:20191108-204452.log

[1] https://bugs.chromium.org/p/chromium/issues/detail?id=1021751
[2] https://chromium.googlesource.com/chromiumos/platform/crosutils/+/master/sdk_lib/make_conf_util.sh

Closes: https://github.com/gentoo/portage/pull/478
Signed-off-by: Edward Baker <edward.baker@intel.com>
Signed-off-by: Zac Medico <zmedico@gentoo.org>
(cherry picked from commit 07f5c5330d4508990ad52e66af4e18f1f860b2eb)

BUG=chromium:1021751
TEST=Verify build_packages completes
     `cros_workon --host start portage`
     `sudo emerge portage`
     `setup_board --board=soraka`
     `./build_packages --board=soraka`

Change-Id: I61d7803e56dd71d16495a58693453e1d2134c71c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/portage_tool/+/1918116
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Edward Baker <edward.baker@intel.com>
1 file changed