theia: set $TMPDIR when installing

installing vscode-ripgrep creates a folder in temporary directory, but
never removes it.

On our test nodes it was observed that a first testnode running installs
and create this folder in /tmp/ then another testnode running as another
unix user installs and we had this error:

    Command: node ./lib/postinstall.js
    Arguments:
    Directory: /srv/slapgrid/slappart13/srv/testnode/cqg/inst/test0-0/tmp/soft/c3f3d686d3e77f9e5c20a555a2698c98/parts/theia/node_modules/vscode-ripgrep
    Output:
    Finding release for v11.0.1-2
    GET https://api.github.com/repos/microsoft/ripgrep-prebuilt/releases/tags/v11.0.1-2
    Downloading from https://api.github.com/repos/microsoft/ripgrep-prebuilt/releases/assets/13800816
    Downloading to /tmp/vscode-ripgrep-cache-1.5.7/ripgrep-v11.0.1-2-x86_64-unknown-linux-musl.tar.gz
    Download options: {"headers":{"user-agent":"vscode-ripgrep","accept":"application/octet-stream"}}
    events.js:167
        throw er; // Unhandled 'error' event
        ^

    Error: EACCES: permission denied, open '/tmp/vscode-ripgrep-cache-1.5.7/ripgrep-v11.0.1-2-x86_64-unknown-linux-musl.tar.gz'
    Emitted 'error' event at:
        at fs.open (internal/fs/streams.js:250:12)
        at FSReqWrap.oncomplete (fs.js:145:20)
    info Visit https://yarnpkg.com/en/docs/cli/install for documentation about this command.
    theia: Non zero exit code (1) while running command.
    While:
    Installing theia.
    Error: Non zero exit code (1) while running command.

/reviewed-on nexedi/slapos!668