runner setting is ignored when set via target.cfg(..)

[japaric]

STR

$ cargo new --bin hello && cd $_

$ mkdir .cargo

$ # this works
$ cat >.cargo/config <<'EOF'
[target.x86_64-unknown-linux-gnu]
runner = "true"
EOF

$ cargo run
   Compiling hello v0.1.0 (file:///home/japaric/tmp/hello)
    Finished dev [unoptimized + debuginfo] target(s) in 0.28s
     Running `true target/debug/hello`

$ # this doesn't
$ cat >.cargo/config <<'EOF'
[target.'cfg(target_os = "linux")']
runner = "true"
EOF

$ cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.00s
     Running `target/debug/hello`
Hello, world!

$ # however rustflags *are* accepted when set via `target.'cfg(..)'`
$ cat >.cargo/config <<'EOF'
[target.'cfg(target_os = "linux")']
rustflags = "--error"
EOF

$ cargo run
   Compiling hello v0.1.0 (file:///home/japaric/tmp/hello)
error: Unrecognized option: 'error'

Meta

$ cargo -V
cargo 1.29.0-nightly (0ec7281b9 2018-08-20)

is there a reason why this doesn't work? or is it a bug? My use case is writing a single target.'cfg(..)' instead of several individual target.$triple settings.