Fix (add|set)env to keep currently set dir for the command, fixes #42131. (#43276)

(cherry picked from commit f53de73)

Author: fredrikekre

base/cmd.jl

@@ -230,7 +230,7 @@ byteenv(env::Union{AbstractVector{Pair{T,V}}, Tuple{Vararg{Pair{T,V}}}}) where {
     String[cstr(k*"="*string(v)) for (k,v) in env]
 
 """
-    setenv(command::Cmd, env; dir="")
+    setenv(command::Cmd, env; dir)
 
 Set environment variables to use when running the given `command`. `env` is either a
 dictionary mapping strings to strings, an array of strings of the form `"var=val"`, or
@@ -239,11 +239,13 @@ existing environment, create `env` through `copy(ENV)` and then setting `env["va
 as desired, or use `addenv`.
 
 The `dir` keyword argument can be used to specify a working directory for the command.
+`dir` defaults to the currently set `dir` for `command` (which is the current working
+directory if not specified already).
 """
-setenv(cmd::Cmd, env; dir="") = Cmd(cmd; env=byteenv(env), dir=dir)
-setenv(cmd::Cmd, env::Pair{<:AbstractString}...; dir="") =
+setenv(cmd::Cmd, env; dir=cmd.dir) = Cmd(cmd; env=byteenv(env), dir=dir)
+setenv(cmd::Cmd, env::Pair{<:AbstractString}...; dir=cmd.dir) =
     setenv(cmd, env; dir=dir)
-setenv(cmd::Cmd; dir="") = Cmd(cmd; dir=dir)
+setenv(cmd::Cmd; dir=cmd.dir) = Cmd(cmd; dir=dir)
 
 """
     addenv(command::Cmd, env...; inherit::Bool = true)

test/spawn.jl

@@ -735,6 +735,25 @@ end
     end
 end
 
+@testset "setenv with dir (with tests for #42131)" begin
+    dir1 = joinpath(pwd(), "dir1")
+    dir2 = joinpath(pwd(), "dir2")
+    cmd = Cmd(`julia`; dir=dir1)
+    @test cmd.dir == dir1
+    @test Cmd(cmd).dir == dir1
+    @test Cmd(cmd; dir=dir2).dir == dir2
+    @test Cmd(cmd; dir="").dir == ""
+    @test setenv(cmd).dir == dir1
+    @test setenv(cmd; dir=dir2).dir == dir2
+    @test setenv(cmd; dir="").dir == ""
+    @test setenv(cmd, "FOO"=>"foo").dir == dir1
+    @test setenv(cmd, "FOO"=>"foo"; dir=dir2).dir == dir2
+    @test setenv(cmd, "FOO"=>"foo"; dir="").dir == ""
+    @test setenv(cmd, Dict("FOO"=>"foo")).dir == dir1
+    @test setenv(cmd, Dict("FOO"=>"foo"); dir=dir2).dir == dir2
+    @test setenv(cmd, Dict("FOO"=>"foo"); dir="").dir == ""
+end
+
 
 # clean up busybox download
 if Sys.iswindows()