Enforce that glue(.envir =) is an environment

[jennybc]

I introduced this in e2b74ff in response to #308.

But this did break some usage in the wild. I currently think those are cases of sub-optimal glue usage and have opened PRs or issues or, in the case of shinyCohortBuilder, sent an email. Here's what that looked like:

https://github.com/tidyverse/glue/blob/c98f106c420d658da8010627d4a2a4d19f97d758/revdep/problems.md

In the interest of doing a glue release, I'm rolling back the check that .envir is an environment, but I would like to bring it back in the future. For follow-up, here are the related issues or PRs, which lays the groundwork for a future release:

• Failure with dev glue rubenarslan/codebook#77
• Prefer glue_data() when data might be list-ish, not an actual environment mrc-ide/odin#310
• Prefer glue_data() when data might be list-ish, not an actual environment sahilseth/params#15
• Prefer glue_data() when the data source is a list bcjaeger/table.glue#1
• Failure with dev glue markfairbanks/tidytable#788
• Failure with dev glue vegawidget/vegawidget#227
• email sent to maintainer of https://github.com/cran/shinyCohortBuilder