Use npx instead of npm exec to support npm 6+

[filmaj]

npm exec is not available in npm v6 (thanks for catching this @srajiang !) so moving to use npx instead, which is supported in older versions of npm.

This PR should be used in conjunction with srajiang/bolt-js-starter-template#2.

[codecov]

Codecov Report

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@@                 Coverage Diff                 @@
##             add-app-function    #1451   +/-   ##
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  Coverage                    ?   79.73%           
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  Files                       ?       20           
  Lines                       ?     1609           
  Branches                    ?      465           
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  Hits                        ?     1283           
  Misses                      ?      220           
  Partials                    ?      106           

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[mwbrooks]

Good catch @srajiang! This worked for me using npm@8.4.1.

[filmaj]

Ah yes I should mention I tested this with node v12.22.12 (npm v6.14.16), node v14.19.2 (npm v6.14.17) and node v16.14.0 (npm v8.3.1) - just based on what we currently have in the "engines" field of bolt's package.json.