indirect installer: Use curl instead of wget

wget 1.12 on Chromium OS < 5116 (R33) is buggy and not able to fetch the latest installer.

Also, use direct URL as curl does not follow redirections.

Fixes #1033 for good, so we can leave the version check in the main installer.
I guess we should, yes. We never use the fancy features of wget anyway (like recursive fetch), and we would save a few chars in some places (`-O-` is the default behaviour of curl).

On this issue, I want to fetch an old stable issue (maybe around 4100) and check that curl works as intended.