fix: Add additional flags for compile

[cpswan]

Suggestion from dart-lang/sdk#54198 (comment) to deal with armv7 build failing on docker buildx emulated armv7

- What I did

Added --extra-gen-snapshot-options=--target_unknown_cpu to dart compile exe line

- How to verify it

Manual run of build workflow

- Description for the changelog

fix: Add additional flags for compile

[cpswan]

Converted to draft as it's still not working:

#22 [linux/arm/v7 build 4/4] RUN dart compile exe --extra-gen-snapshot-options=--target_unknown_cpu   /app/showplatform.dart -o /app/dartshowplatform
#22 5.692 
#22 5.692 ===== CRASH =====
#22 5.692 si_signo=Illegal instruction(4), si_code=2, si_addr=0x3ed8a1c8
#22 5.736 Error: AOT compilation failed
#22 5.737 Generating AOT kernel dill failed!
#22 ERROR: process "/bin/sh -c dart compile exe --extra-gen-snapshot-options=--target_unknown_cpu   /app/showplatform.dart -o /app/dartshowplatform" did not complete successfully: exit code: 64