Often in containers, the files we use to detect whether a cray system supports new features are not available.
Given that the cray containers only support the newer versions, and that these versions have been
around for a while at this point and few sites don't support them, this PR changes the logic for
detecting cray systems so that:
1. Don't even consider whether something is the `cray` platform if `opt/cray` is not in `MODULEPATH`
2. Only use the `cray` platform if we can read files in /opt/cray/pe and positively detect an older version
3. Otherwise, assume we're *not* on a cray (includes newer Cray PE's, which we treat as Linux)
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