build: close sftp connection when done

[holiman]

Some issue with the ppa sftp server makes it not close the connection when the client tries to terminate.

[user@work sftp]$ sftp -i ./ppakey geth-ci@ppa.launchpad.net
Connected to ppa.launchpad.net.
sftp> ls
sftp> exit
Interrupt  
Interrupt  
Interrupt  

More details

sftp> quit
debug2: channel 0: read<=0 rfd 4 len 0
debug2: channel 0: read failed
debug2: chan_shutdown_read: channel 0: (i0 o0 sock -1 wfd 4 efd 6 [write])
debug2: channel 0: input open -> drain
debug2: channel 0: ibuf empty
debug2: channel 0: send eof
debug3: send packet: type 96
debug2: channel 0: input drain -> closed
Interrupt  

Also seen here: kolban-google/sftp-gcs#23 (comment)
Pending fix here: mscdex/ssh2#1111

This PR fixes it, so that when we read the 'sftp> exit' command echoed back on stdout, we terminate on our side. This will cause the Wait to return an error, which we need to ignore to not CRIT the process.