cmd/build-lambda-zip: Produce valid .zips for provided runtime functions

[bmoffatt]

Description of changes:

This is a follow-up to #298, which added support for using Go with the provided runtime. This change updates the build-lambda-zip tool.

In addition to making the first argument executable, a symlink named "bootstrap" will also be placed in the root of the .zip file, and will point to the executable.

doc reference: https://docs.aws.amazon.com/lambda/latest/dg/runtimes-custom.html

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-commenter]

Codecov Report

Merging #307 into master will decrease coverage by 3.59%.
The diff coverage is 44.44%.

@@            Coverage Diff             @@
##           master     #307      +/-   ##
==========================================
- Coverage   77.66%   74.06%   -3.60%     
==========================================
  Files          17       18       +1     
  Lines         761      833      +72     
==========================================
+ Hits          591      617      +26     
- Misses        116      152      +36     
- Partials       54       64      +10     

Impacted Files	Coverage Δ
cmd/build-lambda-zip/main.go	36.11% <44.44%> (ø)

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