custom/ipc: Only first appearance of %pid% is replaced.

[patrick96]

Checklist

• I have read the appropriate section in the contributing
  guidelines
• I believe this issue is a problem with polybar itself and not a misconfiguration on my part.
• I have searched for other open and closed issues that
  may have already reported this problem.
• I have checked the known issues page for this problem.

Describe the bug

Expected behavior:
Using the %pid% token multiple times in the same hook command or click action should replace all tokens.

Actual behavior:

Only the first instance of %pid% in any hook command or click actions is replaced.

Was it working before?

Seems to have always been like this.

To Reproduce

[bar/example]
modules-left = ipctest
enable-ipc = true
[module/ipctest]
type = custom/ipc
hook-0 = echo "%pid% %pid%"
initial = 1

Screenshots

Environment:

• WM: Herbstluftwm
• Distro: Archlinux
• Output of polybar -vvv:

polybar 3.5.6-118-gc65f6857

Features: +alsa +curl +i3 +mpd +network(libnl) +pulseaudio +xkeyboard

X extensions: +randr (+monitors) +composite +xkb +xrm +xcursor

Build type: Release
Compiler: /usr/bin/c++
Compiler flags: -D_FORTIFY_SOURCE=2 -march=x86-64 -mtune=generic -O2 -pipe -fno-plt -Wall -Wextra -Wpedantic -Wsuggest-override -O3 -DNDEBUG
Linker flags: -Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now 

Additional context

The ipc module just searches for the first instance.