Fix logging bug in dragon_install

[AlyssaCote]

I added extra logging depending on the type of mismatch between the asset and the platform. I also return None if the asset is a CRAY EX asset but the platform is not a CRAY EX platform. I broke it down into these potential situations:

is cray ex platform	"crayex" in asset name	Results (asset, log output)
True	False	asset, Platform-specific package not found. Using {asset.name}
True	True	asset, no log output
False	False	asset, no log output
False	True	None, Platform does not support Cray EX assets

NOTE: The string crayex that we check is in the asset name will need to be changed to hsn as soon as Matt's work goes in because the asset names now have hsn instead of crayex.

[codecov]

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