Keybase login fails with GPG Unavailable error

I'm unable to login on recent Ubuntu builds of keybase. The most illuminating lines in the debug output have been:

2017-02-18T13:18:45.984557-08:00 ▶ [DEBU keybase log.go:30] 1d0 - RunEngine(loginProvision) -> ERROR: GPG is unavailable on this device
2017-02-18T13:18:45.984745-08:00 ▶ [DEBU keybase log.go:30] 1d1 - RunEngine(Login) -> ERROR: GPG is unavailable on this device
2

My preferred gpg command is gpg2, full path /usr/bin/gpg2. So far I've tried these variations on the command:
keybase --gpg=gpg2 login
keybase --gpg=/usr/bin/gpg2 login
env GPG=gpg2 keybase login
env GPG=/usr/bin/gpg2 keybase login

All of these attempts end in the error message:

▶ ERROR in Login

Sorry, your account is already established with a PGP public key, but this
utility cannot access the corresponding private key. You need to prove
you're you. We suggest one of the following:

   - install GPG and put your PGP private key on this machine and try again
   - reset your account and start fresh: https://keybase.io/#account-reset

I was going to try to put additional logging in go/libkb/gpg_cli.go and go/libkb/util_nix.go, but I was unable to run packaging/clean_build_kbstage.sh or find working build-from-stock-OS-image directions for the client...

my log id: 0f5d386f56412a9fb37ca71c

Start with go/libkb/gpg_cli.go and go/libkb/util_nix.go, then review the login path that reports “GPG is unavailable on this device.” Reproduce the failure on a recent Ubuntu build using gpg2 and the supplied command variations; packaging/clean_build_kbstage.sh and the missing stock-OS build directions are relevant setup obstacles. Done means the client can access the existing private key and complete login with gpg2.