fix(expand): use transform.Bytes to fix interpolation of large environment variables

[aetheryx]

Originally reported in #115.

Problem

When trying to use environment variable interpolation with large environment variable values, expand fails with couldn't expand environment: transform: short destination buffer. For example:

test:
  value: ${TEST_VALUE}

type TestConfig struct {
  cfgparse.Struct `config:"test"`
  Value           string `yaml:"value"`
}

func main(config TestConfig) {
  fmt.Printf("value: %d\n", len(config.Value))
}

% export TEST_VALUE=$(printf 'a%.0s' {1..4096})
% bazel run 
[...]
value: 4096

% export TEST_VALUE=$(printf 'a%.0s' {1..4097}) # 4096 + 1
[...]
couldn't expand environment: transform: short destination buffer

Changes

Updated expand.go to use transform.Bytes(transformer, bytes) instead of ioutil.ReadAll(transform.NewReader(bytes, transformer)).

The reason for this is that transform.NewReader transforms chunks at fixed sizes, and when an iteration of the transformer returns ErrShortDst without making progress (nDst == 0 && nSrc == 0), NewReader bubbles up ErrShortDst by design and won't grow dst. transform.Bytes does grow dst in this case, as it treats the entire buffer as a whole instead of chunking.

Test plan

1. Unit tests pass
2. Minimum reproducible example from above passes on this branch:

% export TEST_VALUE=$(printf 'a%.0s' {1..100000})
% bazel run 
[...]
value: 100000

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