ffsend: Bump version and add --locked flag to cargo install

[samford]

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Related to #46025 but I also bumped the version to v0.2.55 in the process of adding the --locked flag to cargo install.

[fxcoudert]

Thanks @samford!

PS: when the version change, we want the commit message to be formula x.y.z (even if there are other changes). This makes it easier to grep for version changes in git logs, for one thing.