Allow escaping dollar sign in command and script

[cbandy]

Explanation

I want to pass a literal containing $$ to my command operation as an argument. Unfortunately, Chainsaw removes this value when/before invoking the command.

Related issue

Proposed Changes

This treats pairs of dollar signs as an escape representing one dollar sign. This matches Kubernetes' behavior for container command and args fields.

Checklist

• I have read the contributing guidelines.
• I have read the PR documentation guide and followed the process including adding proof manifests to this PR.
• This is a bug fix and I have added unit tests that prove my fix is effective.

Further Comments

The behavior of os.Expand is undefined and inconsistent for runs of dollar signs U+0024. Without this change, the proposed test fails:

$ go test ./pkg/utils/env
--- FAIL: TestExpand (0.00s)
    --- FAIL: TestExpand/escape (0.00s)
        expand_test.go:45: 
                Error Trace:    .../chainsaw/pkg/utils/env/expand_test.go:45
                Error:          Not equal: 
                                expected: []string{"echo", "DO $ END", "$$", "$literal"}
                                actual  : []string{"echo", "DO  END", "$$$$", "literal"}
                            
                                Diff:
                                --- Expected
                                +++ Actual
                                @@ -2,5 +2,5 @@
                                  (string) (len=4) "echo",
                                - (string) (len=8) "DO $ END",
                                - (string) (len=2) "$$",
                                - (string) (len=8) "$literal"
                                + (string) (len=7) "DO  END",
                                + (string) (len=4) "$$$$",
                                + (string) (len=7) "literal"
                                 }
                Test:           TestExpand/escape
FAIL
FAIL    github.com/kyverno/chainsaw/pkg/utils/env       0.008s
FAIL