allow skipping validation

ci runs in chisel releases need to install slices multiple times. every install runs on the same repo and is forced to run validation. this currently takes about 60% of the CI time. it woudl be an incredibly useful feature to allow us to skip validation in certain contexts. i've been running chisel quite successfully **only for local dev-time testing** with the following:

```diff
diff --git i/internal/setup/setup.go w/internal/setup/setup.go
index 4502687..1909f8e 100644
--- i/internal/setup/setup.go
+++ w/internal/setup/setup.go
@@ -187,6 +187,12 @@ func ReadRelease(dir string) (*Release, error) {
return nil, err
}

+ // skip validation for cached releases is CHISEL_HACKS flag is set
+ if os.Getenv("CHISEL_HACKS") != "" {
+ logf("Skipping release validation due to CHISEL_HACKS")
+ return release, nil
+ }
+
err = release.validate()
if err != nil {
return nil, err

```

*(i don't propose this exact code to be merged)*

- https://github.com/canonical/chisel-releases/pull/580#issuecomment-3148603618