fix(agent): filter out GOTRACEBACK=none (#16924)

With the switch to Go 1.24.1, our dogfood workspaces started setting
`GOTRACEBACK=none` in the environment, resulting in missing stacktraces
for users.

This is due to the capability changes we do when
`USE_CAP_NET_ADMIN=true`.

https://github.com/coder/coder/blob/564b387262e5b768c503e5317242d9ab576395d6/provisionersdk/scripts/bootstrap_linux.sh#L60-L76

This most likely triggers a change in securitybits which sets
`_AT_SECURE` for the process.

https://github.com/golang/go/blob/a1ddbdd3ef8b739aab53f20d6ed0a61c3474cf12/src/runtime/os_linux.go#L297-L327

Which in turn triggers secure mode:

https://github.com/golang/go/blob/a1ddbdd3ef8b739aab53f20d6ed0a61c3474cf12/src/runtime/security_unix.go

This should not affect workspaces as template authors can still set the
environment on the agent resource.

See https://pkg.go.dev/runtime#hdr-Security

agent/agentexec/cli_linux.go

@@ -17,6 +17,8 @@ import (
 	"golang.org/x/sys/unix"
 	"golang.org/x/xerrors"
 	"kernel.org/pub/linux/libs/security/libcap/cap"
+
+	"github.com/coder/coder/v2/agent/usershell"
 )
 
 // CLI runs the agent-exec command. It should only be called by the cli package.
@@ -114,7 +116,8 @@ func CLI() error {
 
 	// Remove environment variables specific to the agentexec command. This is
 	// especially important for environments that are attempting to develop Coder in Coder.
-	env := os.Environ()
+	ei := usershell.SystemEnvInfo{}
+	env := ei.Environ()
 	env = slices.DeleteFunc(env, func(e string) bool {
 		return strings.HasPrefix(e, EnvProcPrioMgmt) ||
 			strings.HasPrefix(e, EnvProcOOMScore) ||