Update Rust plugin to be able to rebuild

[ChrisMacNaughton]

Currently, when a user re-runs snapcraft against a Rust project, the compiler refuses to "re"-install the package, and crashes the build. This change adds the --force flag to the install command which tells the rust compiler to re-install the package. This does not break the cargo compiler cache.

This PR resolves LP: #1825858

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• Have you successfully run ./runtests.sh static?
• Have you successfully run ./runtests.sh tests/unit?

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[sergiusens]

Hi, thanks for this, do you think you could create a new spread test, maybe tests/spread/plugins/rust/rebuild ?

Or if you do not mind, I can do it as well.

[codecov-io]

Codecov Report

Merging #2620 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2620   +/-   ##
=======================================
  Coverage   89.12%   89.12%           
=======================================
  Files         208      208           
  Lines       14207    14207           
  Branches     2157     2157           
=======================================
  Hits        12662    12662           
  Misses       1091     1091           
  Partials      454      454

Impacted Files	Coverage Δ
snapcraft/plugins/rust.py	95.33% <ø> (ø)	⬆️

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[sergiusens]

Thanks!