ZTS: Fix /usr/bin/env: 'python2': No such file or directory

[loli10K]

Motivation and Context

Recently i lost the ability to run the ZTS on my "golden" builder image (Debian9):

root@linux:~# head config.log 
This file contains any messages produced by compilers while
running configure, to aid debugging if configure makes a mistake.

It was created by zfs configure 0.8.0, which was
generated by GNU Autoconf 2.69.  Invocation command line was

  $ ./configure --prefix=/usr --sysconfdir=/etc --libdir=/lib64 --enable-debug --with-python=2.7

## --------- ##
## Platform. ##
root@linux:~# sudo -u nobody -s /usr/share/zfs/zfs-tests.sh -d /var/tmp -v -T removal 

--- Configuration ---
Runfile:         /usr/share/zfs/runfiles/linux.run
STF_TOOLS:       /usr/share/zfs/test-runner
STF_SUITE:       /usr/share/zfs/zfs-tests
STF_PATH:        /var/tmp/constrained_path.aZwC
FILEDIR:         /var/tmp
FILES:           /var/tmp/file-vdev0 /var/tmp/file-vdev1 /var/tmp/file-vdev2
LOOPBACKS:       /dev/loop0 /dev/loop1 /dev/loop2 
DISKS:           loop0 loop1 loop2
NUM_DISKS:       3
FILESIZE:        4G
ITERATIONS:      1
TAGS:            removal
STACK_TRACER:    no
Keep pool(s):    rpool
Missing util(s): arc_summary3 mmap_libaio arp bc fio umask wait 

/usr/share/zfs/test-runner/bin/test-runner.py  -c "/usr/share/zfs/runfiles/linux.run" -T "removal" -i "/usr/share/zfs/zfs-tests" -I "1"
/usr/bin/env: 'python2': No such file or directory
/usr/bin/env: 'python2': No such file or directory
root@linux:~# 

Description

Since 4f342e4 env(1) must be able to find a "python2" executable in the "constrained path" on systems configured with --with-python=2.x otherwise the ZFS Test Suite won't be able to use Python scripts.

How Has This Been Tested?

Tested on Debian9 with the following configuration:

./configure --prefix=/usr --sysconfdir=/etc --libdir=/lib64 --enable-debug --with-python=2.7

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the ZFS on Linux code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• All commit messages are properly formatted and contain Signed-off-by.

[codecov]

Codecov Report

Merging #9325 into master will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9325      +/-   ##
==========================================
+ Coverage   79.02%   79.09%   +0.07%     
==========================================
  Files         401      401              
  Lines      122469   122469              
==========================================
+ Hits        96777    96863      +86     
+ Misses      25692    25606      -86

Flag	Coverage Δ
#kernel	79.77% <ø> (-0.02%)	⬇️
#user	66.8% <ø> (+0.29%)	⬆️

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