bugfix: pouch exec with envs will not replace old envs

[knightXun]

Ⅰ. Describe what this PR did

Bugfix: pouch exec with env will not replace old envs.
This pr is to fix it.
Add a test for this situation.

Ⅱ. Does this pull request fix one issue?

Ⅲ. Why don't you add test cases (unit test/integration test)? (你真的觉得不需要加测试吗？)

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[codecov]

Codecov Report

Merging #2275 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2275      +/-   ##
==========================================
- Coverage    66.8%   66.77%   -0.03%     
==========================================
  Files         208      208              
  Lines       16934    16934              
==========================================
- Hits        11312    11308       -4     
- Misses       4260     4262       +2     
- Partials     1362     1364       +2

Flag	Coverage Δ
#criv1alpha1test	32.63% <100%> (+0.12%)	⬆️
#criv1alpha2test	36.14% <100%> (+0.16%)	⬆️
#integrationtest	39.49% <100%> (-0.05%)	⬇️
#nodee2etest	33.4% <100%> (-0.03%)	⬇️
#unittest	23.75% <0%> (ø)	⬆️

Impacted Files	Coverage Δ
daemon/mgr/container_exec.go	67.28% <100%> (ø)	⬆️
apis/server/utils.go	61.9% <0%> (-4.77%)	⬇️
pkg/meta/store.go	62.5% <0%> (-1.57%)	⬇️
daemon/mgr/container_utils.go	83.13% <0%> (-1.21%)	⬇️
cri/v1alpha2/cri_wrapper.go	60% <0%> (-1.21%)	⬇️
ctrd/container.go	59.28% <0%> (-0.48%)	⬇️
cri/v1alpha2/cri.go	66.76% <0%> (-0.3%)	⬇️
daemon/mgr/container.go	57.39% <0%> (ø)	⬆️
cri/v1alpha1/cri.go	61.88% <0%> (+0.33%)	⬆️
daemon/containerio/container_io.go	77.04% <0%> (+1.09%)	⬆️
... and 2 more

[pouchrobot]

@knightXun Thanks for your contribution. 🍻
Please sign off in each of your commits.

[knightXun]

/cc @fuweid @allencloud

[fuweid]

LGTM