etlas doesn't find custom build tool (alex) in windows when building a package

[jneira]

• Bug discovered trying to patch a package that uses alex: https://github.com/typelead/eta-hackage/pull/149
• eta needs a custom version of alex that doesn't add c headers when generate the .hs file from the template .x
• so there is a patched version in eta-hackage suitable for eta
• etlas builds the package with eta and generate the wrapper scripts in the store (alex in nix and alex.cmd in windows)
• then you can run it with etlas run alex -- --version cause it is aware of windows specific script name
• but when you tries to build the package etlas is not able to find the windows script:

?callStack, called at .\Distribution\Compat\Stack.hs:45:13 in etlas_1lPwCyzvAfM6
mN1zk19Qet:Distribution.Compat.Stack
  callStack, called at .\Distribution\Simple\Utils.hs:592:44 in etlas_1lPwCyzvAf
M6mN1zk19Qet:Distribution.Simple.Utils
  withCallStackPrefix, called at .\Distribution\Simple\Utils.hs:629:7 in etlas_1
lPwCyzvAfM6mN1zk19Qet:Distribution.Simple.Utils
  withMetadata, called at .\Distribution\Simple\Utils.hs:354:15 in etlas_1lPwCyz
vAfM6mN1zk19Qet:Distribution.Simple.Utils
  die', called at .\Distribution\Simple\Program\Db.hs:423:28 in etlas_1lPwCyzvAf
M6mN1zk19Qet:Distribution.Simple.Program.Db
  requireProgram, called at .\Distribution\Simple\Configure.hs:1471:27 in etlas_
1lPwCyzvAfM6mN1zk19Qet:Distribution.Simple.Configure
  configureRequiredProgram, called at .\Distribution\Simple\Configure.hs:1415:10
 in etlas_1lPwCyzvAfM6mN1zk19Qet:Distribution.Simple.Configure
  configureRequiredPrograms, called at .\Distribution\Simple\Configure.hs:594:11
 in etlas_1lPwCyzvAfM6mN1zk19Qet:Distribution.Simple.Configure
  configure, called at .\Distribution\Simple.hs:593:20 in etlas_1lPwCyzvAfM6mN1z
k19Qet:Distribution.Simple
  confHook, called at .\Distribution\Simple\UserHooks.hs:67:5 in etlas_1lPwCyzvA
fM6mN1zk19Qet:Distribution.Simple.UserHooks
  confHook, called at .\Distribution\Simple.hs:224:24 in etlas_1lPwCyzvAfM6mN1zk
19Qet:Distribution.Simple
  configureAction, called at .\Distribution\Simple.hs:184:19 in etlas_1lPwCyzvAf
M6mN1zk19Qet:Distribution.Simple

etlas.exe: The program 'alex' is required but it could not be found.

• So it seems that the functions used to find executables doesn't take in account the windows specific script name
  • If you place a alex.exe in the store alex path it works
• the workaround is set the path to alex.cmd script in etlas global config or as cli param --with-alex path