Try to fix the wasmtime Rust build

[alexcrichton]

This is an attempt to fixup the errors found on #3292. Although I'm not
certain where the error was coming from this switches the Rust
installation to being in PATH by default so there's no need to
source any scripts to get access to the Rust compiler.

cc @fitzgen, @jfoote, y'all are likely interested in this!

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[alexcrichton]

@googlebot I signed it!

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[jfoote]

Thanks @alexcrichton! I continued commenting on this problem over at #3292. Regardless, hopefully this does the trick.

[TravisBuddy]

Hey @alexcrichton,
Your changes look good to me!

View build log

TravisBuddy Request Identifier: c2f69f80-41da-11ea-a094-3d03eb93742b

[jonathanmetzman]

This seemed to fix it!
I wouldn't be shocked if source broke things. One difference between travis and the actual builds is that in travis we use helper.py (as you would locally) and I think travis uses a fake tty. I wonder if either of these affects whether source can be used.