feat: installer downloads and executes dfxvm

[ericswanson-dfinity]

Description

Alters the install script to install dfxvm, rather than dfx.

Note the base branch for the PR is not master. A PR from the base branch will be submitted once we are ready to release dfxvm.

The script is a lot simpler, in part because it always downloads the latest version. Even so, I restructured it a bit:

• make sure the temporary directory is always cleaned up
• handle errors in subshells properly
• never try to download an aarch64-unknown-linux-gnu build, which wouldn't exist

Also updates the downloader to pass --progress-bar to curl, which is much more streamlined. The --progress-bar parameter was added in curl 5.10 in 1999.

Fixes https://dfinity.atlassian.net/browse/SDK-1278

How Has This Been Tested?

bash ./install.sh , with or without DFX_VERSION, run from the public/ directory.

Checklist:

• The title of this PR complies with Conventional Commits.