Check that conda actually activated during install. (#3103)
Check that conda actually activated during install. (#3103)

Check that conda actually activated.

This glues over a rare glitch in conda (#3029, https://github.com/conda/conda/issues/10401)

To make super-extra sure, it uses realpath(3) (https://linux.die.net/man/3/realpath),
and on OS X adds a polyfill to get access to that because macOS doesn't expose that to its shell, weirdly.
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