*: resolve shellcheck errors.

[hexfusion]

Shellcheck v0.5.0 was released in May. This PR squelches a few errors related to SC2230 which are now reported with the latest shellcheck version.

'shellcheck' started at Fri Oct 26 07:26:01 EDT 2018
shellcheck checking failed:
build:18:5: note: which is non-standard. Use builtin 'command -v' instead. [SC2230]
test:237:7: note: which is non-standard. Use builtin 'command -v' instead. [SC2230]
[...]

ref: SC2230

[codecov-io]

Codecov Report

Merging #10216 into master will increase coverage by 0.18%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10216      +/-   ##
==========================================
+ Coverage   71.67%   71.86%   +0.18%     
==========================================
  Files         390      390              
  Lines       36351    36351              
==========================================
+ Hits        26054    26122      +68     
+ Misses       8484     8423      -61     
+ Partials     1813     1806       -7

Impacted Files	Coverage Δ
proxy/grpcproxy/watcher.go	89.79% <0%> (-4.09%)	⬇️
etcdserver/api/v3rpc/watch.go	82.35% <0%> (-1.64%)	⬇️
clientv3/op.go	73.47% <0%> (-0.87%)	⬇️
etcdserver/v3_server.go	79.17% <0%> (-0.46%)	⬇️
clientv3/leasing/kv.go	90.03% <0%> (-0.34%)	⬇️
clientv3/balancer/grpc1.7-health.go	59.01% <0%> (-0.3%)	⬇️
mvcc/watchable_store.go	84.21% <0%> (ø)	⬆️
etcdserver/server.go	74.73% <0%> (+0.21%)	⬆️
etcdserver/api/rafthttp/stream.go	79.82% <0%> (+0.42%)	⬆️
clientv3/snapshot/v3_snapshot.go	66.26% <0%> (+0.8%)	⬆️
... and 17 more

---

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[mitake]

LGTM

[gyuho]

/lgtm

[gyuho-bot]

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