upgrade: bump to Go 1.12.6

[zhuangqh]

Signed-off-by: zhuangqh zhuangqhc@gmail.com

Ⅰ. Describe what this PR did

upgrade go version to 1.12.6. ready for integration with kubernetes 1.14

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 #2909 into master will increase coverage by 0.04%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #2909      +/-   ##
==========================================
+ Coverage   68.79%   68.84%   +0.04%     
==========================================
  Files         291      291              
  Lines       18259    18259              
==========================================
+ Hits        12561    12570       +9     
+ Misses       4243     4239       -4     
+ Partials     1455     1450       -5

Flag	Coverage Δ
#criv1alpha2_test	38.38% <0%> (ø)	⬆️
#integration_test_0	36.22% <0%> (-0.02%)	⬇️
#integration_test_1	35.65% <0%> (ø)	⬆️
#integration_test_2	36.27% <0%> (+0.02%)	⬆️
#integration_test_3	35.73% <0%> (-0.02%)	⬇️
#node_e2e_test	34.25% <0%> (+0.03%)	⬆️
#unittest	28.01% <0%> (ø)	⬆️

Impacted Files	Coverage Δ
ctrd/utils.go	75.97% <0%> (ø)	⬆️
ctrd/client.go	64.65% <0%> (-1.4%)	⬇️
cri/v1alpha2/cri.go	68.81% <0%> (-0.26%)	⬇️
daemon/mgr/container.go	60.35% <0%> (+0.62%)	⬆️
ctrd/container.go	54.3% <0%> (+0.76%)	⬆️
ctrd/watch.go	78.37% <0%> (+5.4%)	⬆️

[fuweid]

LGTM