Make activate script POSIX-compatible.

[ktlim]

The activate script can be run in a number of shells, not just bash, so it should only use POSIX-compatible syntax.

Checklist

• Used a personal fork of the feedstock to propose changes
• Bumped the build number (if the version is unchanged)
• Reset the build number to 0 (if the version changed)
• Re-rendered with the latest conda-smithy (Use the phrase @conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• Ensured the license file is being packaged.

[conda-forge-webservices]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[beckermr]

Thanks KT!

[timj]

Are we going to get the same problem on line 9 of the script?

[github-actions]

Hi! This is the friendly conda-forge automerge bot!

I considered the following status checks when analyzing this PR:

• linter: passed
• azure: passed

Thus the PR was passing and merged! Have a great day!

[ktlim]

Oops, I moved too fast. @timj you should have pushed an additional fix before the bot got to it :)