x/build/env: make sure "lsof" is available on linux-386-sid

[bradfitz]

In #25628, @ianlancetaylor says:

The test will try to run lsof when it finds a leaked FD. That isn't happening on linux-386-sid. Could we get the lsof package installed on that system?

/cc @dmitshur @bcmills @andybons

[bcmills]

CC @toothrot

We made a lot of progress on test flakiness in 1.13, to the extent that #25628 is now one of the most frequent flakes in the builders. It would be nice to get a diagnosis so that we can deflake it.

[bradfitz]

Dockerfile history suggests it's been there since 2017. Confirmed:

bradfitz@go:~$ docker run -ti gcr.io/symbolic-datum-552/linux-x86-sid:latest lsof 
COMMAND PID USER   FD   TYPE DEVICE SIZE/OFF     NODE NAME
lsof      1 root  cwd    DIR   0,39     4096        2 /
lsof      1 root  rtd    DIR   0,39     4096        2 /
lsof      1 root  txt    REG   0,39   175584       26 /usr/bin/lsof
lsof      1 root  mem    REG   0,39   146968       41 /lib/x86_64-linux-gnu/libpthread-2.28.so
lsof      1 root  mem    REG   0,39    14592       39 /lib/x86_64-linux-gnu/libdl-2.28.so
lsof      1 root  mem    REG   0,39   468944       37 /lib/x86_64-linux-gnu/libpcre.so.3.13.3
lsof      1 root  mem    REG   0,39  1824496       35 /lib/x86_64-linux-gnu/libc-2.28.so
lsof      1 root  mem    REG   0,39   155296       33 /lib/x86_64-linux-gnu/libselinux.so.1
lsof      1 root  mem    REG   0,39   165632       31 /lib/x86_64-linux-gnu/ld-2.28.so
lsof      1 root    0u   CHR  136,0      0t0        3 /dev/pts/0
lsof      1 root    1u   CHR  136,0      0t0        3 /dev/pts/0
lsof      1 root    2u   CHR  136,0      0t0        3 /dev/pts/0
lsof      1 root    3r   DIR   0,44        0        1 /proc
lsof      1 root    4r   DIR   0,44        0 44941523 /proc/1/fd
lsof      1 root    5w  FIFO   0,10      0t0 44940094 pipe
lsof      1 root    6r  FIFO   0,10      0t0 44940095 pipe
lsof      7 root  cwd    DIR   0,39     4096        2 /
lsof      7 root  rtd    DIR   0,39     4096        2 /
lsof      7 root  txt    REG   0,39   175584       26 /usr/bin/lsof
lsof      7 root  mem    REG   0,39   146968       41 /lib/x86_64-linux-gnu/libpthread-2.28.so
lsof      7 root  mem    REG   0,39    14592       39 /lib/x86_64-linux-gnu/libdl-2.28.so
lsof      7 root  mem    REG   0,39   468944       37 /lib/x86_64-linux-gnu/libpcre.so.3.13.3
lsof      7 root  mem    REG   0,39  1824496       35 /lib/x86_64-linux-gnu/libc-2.28.so
lsof      7 root  mem    REG   0,39   155296       33 /lib/x86_64-linux-gnu/libselinux.so.1
lsof      7 root  mem    REG   0,39   165632       31 /lib/x86_64-linux-gnu/ld-2.28.so
lsof      7 root    4r  FIFO   0,10      0t0 44940094 pipe
lsof      7 root    7w  FIFO   0,10      0t0 44940095 pipe