[RaphaelFakhri]

Motivation

swiftly init adds . env.sh to ~/.profile for bash users without ~/.bash_profile or ~/.bash_login (the Debian and Ubuntu default) and for shells other than bash, zsh, and fish. POSIX sh login shells also read ~/.profile. On Debian and Ubuntu, /bin/sh is dash, which doesn't support the ${PATH//...} and ${PATH/#...} expansions that env.sh uses. dash reports Bad substitution and exits, so the rest of ~/.profile and the calling script don't run:

$ sh -c '. ~/.local/share/swiftly/env.sh; echo "$PATH"'
sh: 6: /root/.local/share/swiftly/env.sh: Bad substitution
$ echo $?
2

The substring removal also isn't anchored on both sides, so a PATH entry such as $SWIFTLY_BIN_DIR2 turns into 2.

Changes

• Generate env.sh with POSIX parameter expansions that remove only whole PATH entries equal to SWIFTLY_BIN_DIR, then prepend it. The temporary variable is unset afterwards.
• Recognize the previous env.sh format during the swiftly init upgrade path, so existing installs get the fixed file.
• Add initEnvShIsPOSIXCompatible, which sources the generated env.sh with /bin/sh and checks the exit status and resulting PATH.
• Add the previous format as a case in initUpgrade and update the expected output.

Testing

• swift test --filter InitTests on Debian 13 (/bin/sh is dash), Swift 6.4.0: fails without the source change (initEnvShIsPOSIXCompatible and three initUpgrade cases) and passes with it (9 tests).
• Sourced the new snippet with dash, bash, and busybox sh for PATH values with the bin directory at the start, middle, and end, repeated, absent, alone, empty, and with a similarly prefixed entry.
• swift run swiftformat --lint --dryrun reports no changes.

Fixes #617