Set correct PATH for exec form

[aca]

Fixes #1304

Go exec.Command return error immediately if it fail to find executable in PATH.
https://github.com/golang/go/blob/dd150176c3cc49da68c8179f740eadc79404d351/src/os/exec/exec.go#L169-L182

In case of exec form in Dockerfile,

kaniko/pkg/commands/run.go

Line 86 in dd4191a

cmd := exec.Command(newCommand[0], newCommand[1:]...)

This commands would fail (in most cases) as we didn't set PATH environment for command yet.

Simplest Dockerfile to reproduce issue.

FROM golang:1.14.2-alpine
COPY . .
RUN ["go", "run", "main.go"]

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Release Notes

- Set correct PATH for exec form