Would like way for top-level tools to not change environment file after setup

The use of the top-level tools for automating things like builds and bldenv is nice. However, they generally rewrite the environment file. While I can understand why this might be helpful, there are a couple of cases where there might be gotchas here (for reference we did this in smartos-live only when someone reran ./configure not when they did a build). In particular here are a few things to think about:

• Someone updates helios build but not the various dependencies. For example, with #3, that person would suddenly find themselves with a broken build.
• If you want to manually control and do a debug-only build (e.g. my default development build is quick, but not multi-proto), you might manually edit the files, but then that'll get undone.
• Makes it hard to test new stuff or potentially lie about things like VERSION for wsdiff.

I'm a little torn on this as I realize that most folks will probably be well served here by the current defaults and it might just be that I'm in a power user camp where I should just use the underlying tools, but I figured I should mention this.

The issue names no files, tests, or entry points. Start by locating the top-level build tools and the path where they rewrite the environment file. Before implementation, define whether builds preserve manual edits or provide an opt-out; done requires agreed behavior covering defaults, overrides, and reruns.