Fix tiup update --self with offline package

[lucklove]

Fix #658

Signed-off-by: lucklove gnu.crazier@gmail.com

Fix the issue that `tiup update --self` may make root.json invalid with offline mirror

[codecov-commenter]

Codecov Report

Merging #659 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #659      +/-   ##
==========================================
- Coverage   57.35%   57.35%   -0.01%     
==========================================
  Files         243      243              
  Lines       17516    17518       +2     
==========================================
  Hits        10047    10047              
- Misses       6167     6169       +2     
  Partials     1302     1302              

Flag	Coverage Δ
#coverage	57.35% <ø> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
go/src/github.com/pingcap/tiup/pkg/utils/ioutil.go	47.54% <0.00%> (-1.64%)	⬇️
...src/github.com/pingcap/tiup/pkg/environment/env.go	37.76% <0.00%> (-0.41%)	⬇️
...c/github.com/pingcap/tiup/pkg/cluster/api/pdapi.go	59.75% <0.00%> (+0.60%)	⬆️

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[lonng]

/merge

[ti-srebot]

/run-all-tests

[ti-srebot]

/run-all-tests

[lonng]

@lucklove Please write the release note for this PR.