keybase update computes wrong prefix on windows

when keybase update the prefix of npm is calculated wrongly

info: Computed npm install prefix: C:\Program
info: Found keybase-installer: keybase-installer v1.0.3
info: Detected custom path 'C:\Program Files\nodejs\node_modules\keybase'; preserving it!
info: Attempting software upgrade.....
info: Running `keybase-installer -g gpg2 --prefix C:\Program Files\nodejs\node_modules\keybase`
warn: Created install directory: C:\Program
info: Made temporary directory: C:\Users\User\AppData\Local\Temp\keybase_install_bNWHbesj5zyHCUF1BEBSjQ
info: Using keyset version v1
info: Fetching URL https://dist.keybase.io/sig/files/1/index.asc
info: Fetching URL https://dist.keybase.io/pkg/Files\nodejs\node_modules\keybase
info: cleaning up tmpdir C:\Users\Florian\AppData\Local\Temp\keybase_install_bNWHbesj5zyHCUF1BEBSjQ
error: In https://dist.keybase.io/pkg/Files/nodejs/node_modules/keybase: HTTP failure, code 404
error: exit code 4294967294

Start with the keybase update entry point and the keybase-installer invocation shown in the report, reproducing the npm prefix calculation on Windows. Check why C:\Program Files\nodejs\node_modules\keybase becomes C:\Program, then verify that the complete prefix is passed through and the update no longer requests the malformed URL.