Use a unique build environment for each platform in a charm recipe. ### Scenario 1 * Fixes a bug where multiple platforms would build in the same build environment if the platforms had the same `build-on` and `build-for` entries. ```yaml platforms: noble: build-on: [riscv64] build-for: [riscv64] noble-debug: build-on: [riscv64] build-for: [riscv64] ``` ### Scenario 2 * Improves build time for multi-base recipes. Previously, Charmcraft would tear down and recreate the same container when building for a different base if the platforms had the same `build-on` and `build-for` entries. ```yaml platforms: jammy: build-on: [ubuntu@22.04:riscv64] build-for: [ubuntu@22.04:riscv64] noble: build-on: [ubuntu@24.04:riscv64] build-for: [ubuntu@24.04:riscv64] ``` Fixes #2058 (CRAFT-3847)