Pin rust version with toolchain file

[kl]

As part of enabling reproducible builds we need to build a given commit with a pinned Rust version.

This commit adds a minimal rust-toolchain.toml file that specifies this version (this Rust version is enforced by rustup when invoking a cargo command).

In the container we extract the version that is specified in rust-toolchain.toml to ensure that the container is pre-built with the correct Rust version (so that when buildling in the container we do not need to download the right Rust version).

This PR sets the Rust version to 1.83.0.

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This change is 

[linear]

DES-554 Pin Rust version and allow automatic install of right version

[albin-mullvad]

Reviewed 3 of 5 files at r1.
Reviewable status: 3 of 5 files reviewed, 1 unresolved discussion (waiting on @kl)

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building/build-and-publish-container-image.sh line 38 at r1 (raw file):

full_container_name="$REGISTRY_HOST/$REGISTRY_ORG/$container_name"

RUST_VERSION=$(rg -o "channel = \"(.*)\"" -r '$1' "$REPO_DIR/rust-toolchain.toml")

We need to do the same in android/fdroid-build/init.sh

Code quote:

RUST_VERSION=$(rg -o "channel = \"(.*)\"" -r '$1' "$REPO_DIR/rust-toolchain.toml")

[albin-mullvad]

Reviewable status: 2 of 8 files reviewed, all discussions resolved

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building/build-and-publish-container-image.sh line 38 at r1 (raw file):

Previously, albin-mullvad wrote…

We need to do the same in android/fdroid-build/init.sh

NVM, that should be handled by the combination of the rust-toolchain.toml file and the init script adding targets during the build process.

[albin-mullvad]

Reviewed 1 of 5 files at r1, 1 of 4 files at r2.
Reviewable status: 4 of 8 files reviewed, all discussions resolved

[faern]

Reviewable status: 4 of 8 files reviewed, 1 unresolved discussion (waiting on @kl)

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a discussion (no related file):
We probably need to update a few more CI workflows. Search for rustup default in the .github/ directory. All those CI jobs set up a default toolchain and expect all subsequent Rust invocation to use that toolchain. But rust-toolchain.toml will override it. This has to be solved in a different way.

We do want to continue running CI against beta and nightly. So those CI jobs need have their overrides fixed to work. Maybe they can just locally delete rust-toolchain.toml before building anything?