rust plugin crashes when trying to rebuild a snap package

[shahn]

To reproduce:

- create a rust project that will build and install executables (e.g. with `cargo init --bin`)
- add a snapcraft.yaml using the rust plugin
- run `snapcraft` (this works)
- run `snapcraft` again. This fails with:

```
/root/.cargo/bin/cargo +stable install --path /root/parts/scriptkeeper/build --root /root/parts/scriptkeeper/install
  Installing scriptkeeper v0.1.0 (/root/parts/scriptkeeper/build)
error: binary `scriptkeeper` already exists in destination as part of `scriptkeeper v0.1.0 (/root/parts/scriptkeeper/build)`
Add --force to overwrite
```

It seems as if the plugin runs `cargo install` on the same virtual machine, but that fails the second time, because `cargo` prevents you from overwriting executables without passing `--force`. `snapcraft clean scriptkeeper` fixes this, but it also means that I have to rebuild the entire project for every little change. Which is annoying when trying to debug the snap file.

Here's the project and commit where I'm seeing this: https://github.com/Originate/scriptkeeper/commit/2160eeeada5a44546e743f11a90798c8c19083e5

Using:
- snapcraft, version 3.4
- multipass 0.6.0
- multipassd 0.6.0
- Ubuntu 18.10
- base: core18

[Kenneth Koski (knkski)]

I found this workaround that allows not doing a full rebuild:

parts:
  part-name:
    source: .
    plugin: rust
    # https://forum.snapcraft.io/t/issue-with-rebuilding-a-rust-snap/10830
    override-build: |
      [ -f "$SNAPCRAFT_PART_INSTALL/bin/juju-kubectl" ] && rm "$SNAPCRAFT_PART_INSTALL/bin/juju-kubectl"
      snapcraftctl build

[Leonora Tindall (lfstindall)]

A potential fix is to add --force to the cargo arguments.

[Chris MacNaughton (chris.macnaughton)]

Fix proposed at https://github.com/snapcore/snapcraft/pull/2620