v0.2.1

Features

• Expand $VAR and ${VAR} in env: front matter values, matching what path:
  entries already did. Front matter can now name one exact executable instead of
  relying on a PATH lookup that may resolve a different build:

  env:
    TOOL: $TRYSCRIPT_GIT_ROOT/target/debug/tool$TRYSCRIPT_EXE

  $$ produces a literal $, so a value that has to keep one, such as a password or a
  sed expression, is not substituted away.

• Add TRYSCRIPT_EXE, the platform executable suffix: .exe on Windows and empty
  elsewhere. It keeps a binary path in front matter portable across platforms.

Fixes

• Expand variables in a single pass. A ${VAR} whose value itself contained $NAME was
  expanded a second time, so a path: entry could resolve somewhere other than the
  directory it named.