CARGO_BUILD_TARGET is not used for cargo-espflash

[diekleinekuh]

Bug description

The CARGO_BUILD_TARGET environment variable is not used by cargo-espflash. It requires the --target command line parameter to be set.

• I have searched the existing issues (closed & open) to make sure the issue hasn't already been reported
• Check this box if you would like to work on a fix

To Reproduce

Steps to reproduce the behavior:

1. Use a riscv esp32 chip like esp32c6 connected via usb
2. Do CARGO_BUILD_TARGET=riscv32imac-esp-espidf cargo espflash flash
3. Observe the output

[2026-04-03T13:34:07Z INFO ] Serial port: '/dev/ttyACM0'
[2026-04-03T13:34:07Z INFO ] Connecting...
[2026-04-03T13:34:07Z INFO ] Using flash stub
Error: cargo_espflash::no_target

  × Failed to build project
  ╰─▶ No target specified in cargo configuration
  help: Specify the target in `.cargo/config.toml`, the esp32c6 support the following targets: riscv32imac-esp-espidf,
        riscv32imac-unknown-none-elf

• I have reproduced the bug in main branch too

Expected behavior

cargo-espflash should honor CARGO_BUILD_TARGET and behave as if --target=riscv32imac-esp-espidf was supplied.

Environment

• OS: Ubuntu 25.10 hosting docker image espressif/idf-rust:esp32_1.92.0.0
• espflash/cargo-espflash version: 4.3.0
• Target: ESP32-C6

Additional context

I'm using different devcontainer configs for targeting esp32 and esp32c6. In those configs I set CARGO_BUILD_TARGET accordingly. cargo build uses the env variable so it would be helpful if I could also use the same command for flashing without providing --target every time.