cmd: use SHELL from environment for completion

[dveeden]

What problem does this PR solve?

tiup completion doesn't auto-detect the shell based on $SHELL and doesn't accept /bin/bash as a value.

What is changed and how it works?

Check List

Tests

• Manual test (add detailed scripts or steps below)

Run tiup completion bash and tiup completion

Release notes:

NONE

[codecov-commenter]

Codecov Report

Merging #1455 (83f936b) into master (0edbf77) will increase coverage by 20.39%.
The diff coverage is 0.00%.

❗ Current head 83f936b differs from pull request most recent head e784286. Consider uploading reports for the commit e784286 to get more accurate results

@@             Coverage Diff             @@
##           master    #1455       +/-   ##
===========================================
+ Coverage   24.66%   45.05%   +20.39%     
===========================================
  Files         267      280       +13     
  Lines       21368    22841     +1473     
===========================================
+ Hits         5270    10292     +5022     
+ Misses      15292    10897     -4395     
- Partials      806     1652      +846     

Flag	Coverage Δ
cluster	32.07% <ø> (?)
dm	24.74% <ø> (?)
integrate	41.35% <0.00%> (+26.23%)	⬆️
tiup	15.14% <0.00%> (+0.02%)	⬆️
unittest	21.30% <0.00%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
cmd/completion.go	19.84% <0.00%> (-0.31%)	⬇️
pkg/cluster/operation/scale_in.go	50.00% <0.00%> (ø)
pkg/cluster/operation/check.go	0.00% <0.00%> (ø)
pkg/cluster/operation/operation.go	69.56% <0.00%> (ø)
pkg/cluster/operation/upgrade.go	23.63% <0.00%> (ø)
pkg/cluster/operation/telemetry.go	0.00% <0.00%> (ø)
pkg/logger/logger.go	100.00% <0.00%> (ø)
components/dm/task/update_dm_meta.go	85.29% <0.00%> (ø)
pkg/logger/audit.go	53.33% <0.00%> (ø)
pkg/cluster/operation/action.go	62.02% <0.00%> (ø)
... and 167 more

---

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[AstroProfundis]

/merge

[ti-chi-bot]

@AstroProfundis: /merge in this pull request requires 1 approval(s).

In response to this:

/merge

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[ti-chi-bot]

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[AstroProfundis]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: e784286