### Please describe the problem.
I can't launch the Assistant on the latest OSX build. Also mentioned by a user at [install/OSX](http://git-annex.branchable.com/install/OSX/).

I can launch the assistant fine with `6.20180409-gfb0780266`

### What steps will reproduce the problem?
Download git-annex.dmg (6.20180430-g393fc79d5) and copy to /Applications folder

Add `/Applications/git-annex.app/Contents/MacOS` to bash path

Run `git-annex-webapp` from command-line:

    andrew@bumblebee ~$ git-annex-webapp
    andrew@bumblebee ~$ uname: illegal option -- o
    usage: uname [-amnprsv]
    git-annex: user error (uname ["-o"] exited 1)
    WebApp crashed: user error (uname ["-o"] exited 1)
    [2018-05-04 09:32:07.67968] WebApp: warning WebApp crashed: user error (uname ["-o"] exited 1)

### What version of git-annex are you using? On what operating system?
macOS Sierra 10.12.6

git-annex version: 6.20180430-g393fc79d5
build flags: Assistant Webapp Pairing S3(multipartupload)(storageclasses) WebDAV FsEvents ConcurrentOutput TorrentParser MagicMime Feeds Testsuite
dependency versions: aws-0.17.1 bloomfilter-2.0.1.0 cryptonite-0.23 DAV-1.3.1 feed-0.3.12.0 ghc-8.0.2 http-client-0.5.7.0 persistent-sqlite-2.6.2 torrent-10000.1.1 uuid-1.3.13 yesod-1.4.5
key/value backends: SHA256E SHA256 SHA512E SHA512 SHA224E SHA224 SHA384E SHA384 SHA3_256E SHA3_256 SHA3_512E SHA3_512 SHA3_224E SHA3_224 SHA3_384E SHA3_384 SKEIN256E SKEIN256 SKEIN512E SKEIN512 BLAKE2B256E BLAKE2B256 BLAKE2B512E BLAKE2B512 BLAKE2B160E BLAKE2B160 BLAKE2B224E BLAKE2B224 BLAKE2B384E BLAKE2B384 BLAKE2S256E BLAKE2S256 BLAKE2S160E BLAKE2S160 BLAKE2S224E BLAKE2S224 BLAKE2SP256E BLAKE2SP256 BLAKE2SP224E BLAKE2SP224 SHA1E SHA1 MD5E MD5 WORM URL
remote types: git gcrypt p2p S3 bup directory rsync web bittorrent webdav adb tahoe glacier ddar hook external

### Please provide any additional information below.

[[!format sh """
# If you can, paste a complete transcript of the problem occurring here.
# If the problem is with the git-annex assistant, paste in .git/annex/daemon.log

andrew@bumblebee ~$ which git-annex-webapp
/Applications/git-annex.app/Contents/MacOS/git-annex-webapp
andrew@bumblebee ~$ git-annex version
git-annex version: 6.20180430-g393fc79d5
build flags: Assistant Webapp Pairing S3(multipartupload)(storageclasses) WebDAV FsEvents ConcurrentOutput TorrentParser MagicMime Feeds Testsuite
dependency versions: aws-0.17.1 bloomfilter-2.0.1.0 cryptonite-0.23 DAV-1.3.1 feed-0.3.12.0 ghc-8.0.2 http-client-0.5.7.0 persistent-sqlite-2.6.2 torrent-10000.1.1 uuid-1.3.13 yesod-1.4.5
key/value backends: SHA256E SHA256 SHA512E SHA512 SHA224E SHA224 SHA384E SHA384 SHA3_256E SHA3_256 SHA3_512E SHA3_512 SHA3_224E SHA3_224 SHA3_384E SHA3_384 SKEIN256E SKEIN256 SKEIN512E SKEIN512 BLAKE2B256E BLAKE2B256 BLAKE2B512E BLAKE2B512 BLAKE2B160E BLAKE2B160 BLAKE2B224E BLAKE2B224 BLAKE2B384E BLAKE2B384 BLAKE2S256E BLAKE2S256 BLAKE2S160E BLAKE2S160 BLAKE2S224E BLAKE2S224 BLAKE2SP256E BLAKE2SP256 BLAKE2SP224E BLAKE2SP224 SHA1E SHA1 MD5E MD5 WORM URL
remote types: git gcrypt p2p S3 bup directory rsync web bittorrent webdav adb tahoe glacier ddar hook external
andrew@bumblebee ~$ git-annex-webapp
andrew@bumblebee ~$ uname: illegal option -- o
usage: uname [-amnprsv]
git-annex: user error (uname ["-o"] exited 1)
WebApp crashed: user error (uname ["-o"] exited 1)
[2018-05-04 09:32:07.67968] WebApp: warning WebApp crashed: user error (uname ["-o"] exited 1)

# End of transcript or log.
"""]]

### Have you had any luck using git-annex before? (Sometimes we get tired of reading bug reports all day and a lil' positive end note does wonders)

Yup! git-annex is great! Thanks for all your hard work on this project Joey!!

—Andrew

> [[fixed|done]] --[[Joey]]
